r~^' 


FIRST  BOOK  OF 


CAL  GEOGRAPHY 


R.,S/TARR 


IN   MEMORIAM 
BERNARD  MOSES 


^ 


^ 


Digitized  by  the  Internet  Archive 

in  2008  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/firstbookofphysiOOtarrrich 


FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 


J^^o 


■s9         ^    O 


Frontispiece. 
Granite  peaks  in  the  Yosemite. 


FIRST   BOOK 


OF 


PHYSICAL    GEOGRAPHY 


BY 


RALPH   S.   TARR,  B.S.,  F.G.S.A. 

PROFESSOR  OF  DYNAMIC   GBOLOGY  AND    PHYSICAL   GBOGRAPHY  AT 

CORNELL   UNIVERSITY 

AUTHOR   OF   "economic   GEOLOGY   OF  THE   UNITED   STATES" 

"  ELEMENTARY    PHYSICAL    GEOGRAPHY  " 

"elementary   geology,"    ETC. 


THE    MACMILLAN   COMPANY 

LONDON:  MACMILLAN  &  CO.,  Ltd. 
1900 

AU  righta  reserved 


T3Z 


BERNARD  MOSES 

Copyright,  1S97, 
By  the  MACMILLAN  COMPANY. 


Set  up  and  electrotyped  June,  1897.       Reprinted   July,   August, 
September,  October,  1897;    March,  July,  1898  ;  March,  1899  ;  August 
1899;  September,  1899;  July-  September,  1900, 


Norhjooti  ^re»8 

J.  S.  Cushin}?  &  Co.  -  Berwick  St  Smith 
Norwood  Mass.  U.S.A. 


PREFACE 

In  preparing  my  Elementary  Physical  Geography,  I 
attempted  to  present  the  subject  in  such  a  way  as  to  put 
it  forward  in  its  more  modern  aspect,  and  particularly  to 
include  the  new  physiography,  or  science  of  land  form. 
An  effort  was  made  to  cover  the  entire  ground  in  a  very 
elementary  way ;  but  at  once  the  difficulty  arose  that  the 
field  was  so  large,  that  even  to  present  the  subject  in 
an  elementary  manner  would  require  a  good-sized  book. 
To  avoid  this  there  were  two  things  possible  :  one  to 
omit  some  parts  and  curtail  others ;  the  other  to  com- 
mence the  consideration  of  some  subjects  with  the  assump- 
tion that  either  the  scholars  knew  the  preliminary  points, 
or  that  the  teacher  could  explain  them.  Both  of  these 
methods  of  shortening  the  book  were  followed,  but  even 
then  it  grew  to  a  size  entirely  too  large  for  those  schools 
in  which  the  subject  has  a  minor  place. 

The  result  has  been,  that  although  this  book  has  met 
with  a  success  wholly  unexpected,  many  teachers  who 
wish  to  give  instruction  in  the  new  physical  geography^ 
are  unable  to  make  use  of  it.  It  is  partly  in  the  hope  of 
meeting  the  needs  of  these,  that  I  have  undertaken  the  prep- 
aration of  this  smaller  book,  from  which  still  other  sub- 
jects are  omitted,  but  in  which  the  attempt  is  made  to  start 
from  the  beginning,  and  make  every  topic  thoroughly  clear, 
assuming  no  more  than  is  absolutely  necessary  in  a  subject 

yii 

787709 


viii  PREFACE 

which  is  based  in  part  upon  certain  well-known  principles 
of  other  scien"Ces,  notably  physics  and  geology. 

If  I  have  been  successful  in  this  effort,  the  other  object 
that  I  have  in  mind  may  also  be  accomplished.  Now,  in 
many  of  the  better  schools,  geography  is  taught  first  as  a 
home  study  of  observation,  and  this  is  followed  by  general 
geography,  and  this  by  physical  geography.  But  the  latter 
subject,  as  treated  in  most  geographies,  is  entirely  inade- 
quate, and  ought  to  be  supplemented,  or  better  still  be 
entirely  replaced,  by  a  real  study  of  physical  geography  in 
the  upper  grade  of  the  grammar  school.  Since  the  great 
majority  of  our  youth  never  go  further  than  the  grammar 
school,  I  believe  that  they  ought  not  to  be  allowed  to  go 
out  into  life  without  a  genuine  knowledge  of  the  main 
principles  of  air  and  earth  sciences.  I  feel  this  strongly, 
not  merely  because  of  the  information  which  they  gain, 
but  also  because  of  the  discipline  and  culture  which  these 
sciences  can  impart. 

In  reality,  it  is  for  the  last  object  that  I  chiefly  seek,  for 
I  believe  tliat  with  this  information  and  training,  the  stu- 
dent, if  he  enters  the  high  school,  will  then  be  able  to  go 
on  with  much  greater  profit  with  a  more  thorough  study 
of  the  earth  sciences,  geology  and  physiography.  Indeed, 
I  would  urge,  that  where  the  general  study  of  physical 
geography  as  a  whole  can  be  put  into  the  grammar  schools, 
the  higli  schools  should  take  up,  not  a  more  thorough  study 
of  the  same  subject,  on  the  "  concentric  "  plan,  but  a  fuller 
study  of  a  part,  or  better  still,  several  parts,  like  meteo- 
rology, physiography,  and  geology. 

Although  having  in  mind  the  use  of  this  book  in  the 
high  school,  I  would  frankly  say  that  I  have  no  sympathy 
with  the  conditions  which  seem  to  demand  it.     Modern 


PREFACE  ix 

education  should  rise  above  fourteen- week  courses,  and  it 
is  better  to  omit  physical  geography,  giving  its  place  to 
a  more  thorough  study  of  some  other  science,  than  to  keep 
it  on  that  plane.  By  peculiar  merits  of  its  own  it  demands 
fuller  study.  It  is  my  hope,  therefore,  that  where  this  book 
may  be  used  in  high  schools  in  which  conditions  have 
made  necessary  a  short  course,  it  may  help  to  create  a  de- 
mand for  better  and  fuller  instruction  in  the  subject. 

Much  of  the  value  of  the  new  physical  geography,  —  in- 
deed I  might  almost  say  its  whole  value,  aside  from  the 
information  which  it  imparts,  —  depends  upon  the  way  in 
which  it  is  taught.  To  assign  certain  pages  to  be  memo- 
rized, and  to  stop  there,  whether  this  is  done  in  the  gram- 
mar or  the  high  school,  is  to  fail  to  obtain  the  full  results 
which  can  be  gained  from  the  study.  Simple  class-room 
experiments  ;  laboratory  study  of  specimens,  maps  and  pho- 
tographs ;  observations  made  by  students  independently, 
and  discussed  in  the  class  ;  practice  in  grouping  facts  which 
logically  lead  to  conclusions ;  and  collective  study  out-of- 
doors,  should  take  the  place  of  much  of  the  time-honored 
recitation.  No  specific  suggestions  are  made  here,  but 
teachers  will  find  some  in  my  earlier  book.  No  school  is 
so  unfavorably  situated  that  opportunities  for  such  study 
cannot  be  found  in  abundance ;  no  genuine  teacher  will 
fail  to  find  pleasure  from  the  results  of  such  study;  and 
no  student  will  fail  to  gain  great  profit  from  it. 

Much  good  comes  in  any  study  when  the  desire  is  created 
for  fuller  information ;  and  still  more  benefit  arises  when 
to  satisfy  this  desire,  students  are  taught  that  there  are 
places  in  which  to  look,  and  are  given  instruction  how  to 
find  them.  To  make  these  benefits  possible,  I  appended 
at  the  close  of  each  chapter  of  my  Elementary  Physical 


X  PREFACE 

Geography,  a  list  of  books  of  reference  selected  from  the 
best.  It  is  not  worth  while  to  repeat  these  here ;  but  at 
the  end  of  this  book  there  are  a  few  supplementary  refer- 
ences, chiefly  to  works  which  have  appeared  since  the 
former  book  was  published. 

This  book  follows  approximately  the  same  order  as  that 
of  my  Elementary  Physical  Geography,  because  I  believe 
that  this  is  the  best.  Still  some  will  be  found  who  prefer 
a  different  order,  and  there  is  no  reason  why  a  teacher 
who  prefers  to  commence  with  the  land  ma}''  not  do  so. 
As  in  my  Elementary  Physical  Geography  and  Elementary 
Geology,  I  have  introduced  illustrations  profusely,  because 
it  is  my  belief  that  next  to  nature  itself,  such  illustrations 
are  of  most  value.  More  can  be  told  in  the  space  occupied 
by  them  than  in  several  times  that  much  space  in  type, 
and  it  can  be  told  more  clearly.  Moreover,  some  of  them 
can  be  made  to  serve  as  a  basis  for  observation  study.  A 
considerable  majority  of  the  illustrations  are  original,  but 
some  are  copied,  and  some  are  reproduced  from  photographs 
taken  by  others.  To  those  who  have  kindly  allowed  me 
to  use  these  I  return  especial  thanks. 

RALPH   S.   TARR. 
Ithaca,  N.  Y.,  June  1,  1897. 


CONTENTS 

Part  I.     Introduction 
CHAPTER   I.     Condition  of  the  Earth 

PAGK 

Form  of  the  Earth 3 

The  Earth  a  Sphere 3 

Longitude 4 

Latitude 6 

The  Earth  an  Oblate  Spheroid 6 

General  Condition  of  the  Earth 7 

Air 7 

Ocean  .        .         . 7 

The  Solid  Earth 8 

Surface  of  the  Earth 9 

Continents  and  Ocean  Basins 9 

Mountain  Irregularities 12 

Minor  Irregularities 13 

Movements  of  the  Earth 13 

Rotation 13 

Revolution 15 

The  Sun  in  the  Heavens 15 

Cause  of  Seasons 17 

CHAPTER   IL     The  Universe 

The  Solar  System 22 

The  Sun 22 

The  Planets .        .        .23 

Satellites 24 

The  Universe 25 

The  Nebular  Hypothesis 27 

Symmetry  of  the  Solar  System 27 

The  Explanation 28 

Facts  accounted  for 30 

Other  NebulsB 31 

zi 


Xll  ^  CONTENTS 

-Part  II.     The  Atmosphere 
CHAPTER   III.     General  Features  of  xiiii  Am 

PAGK 

Importance  of  the  Air 32 

Composition 33 

Oxygen  and  Nitrogen 33 

Carbonic  Acid  Gas 33 

Water  Vapor 35 

Dust  Particles 38 

Height  of  the  Atmosphere 40 

Changes  in  the  Air 42 

CHAPTER   IV.    Light,  Electricity,  and  Magnetism 

Light 

Nature  of  Light 43 

Reflection 44 

Absorption 47 

Selective  Scattering •    , 48 

Refraction 48 

The  Colors  of  Sunrise  and  Sunset 49 

The  Rainbow 51 

Halos  and  Coronas .51 

Sunlight  Measurements 62 

Electricity  and  Magnetism 

Lightning 52 

Magnetism 53 

CHAPTER  V.     Sun's  Heat 

Nature  of  Heat 55 

Reflection  of  Heat 55 

Absorption  of  Heat 57 

Radiation  of  Heat 57 

Conduction  of  Heat 59 

Convection 59 

Heat  on  the  Land 60 


CONTENTS  xili 

PAGK 

Warming  of  the  Ocean 61 

Temperature  of  Highlands 62 

Effect  of  Heat  on  the  Air 64 

Effect  of  Rotation 66 

Effect  of  Revolution 67 

Temperature  Measurement 68 

CHAPTER  VI.      Temperature  of  the  Earth's  Surface 

Day  and  Night  Change 70 

Daily  Range .70 

Change  with  the  Seasons 71 

Effect  of  Land  and  Water .72 

Irregular  Changes 73 

Seasonal  Temperature  Change 74 

Seasonal  Range 74 

Influence  of  Latitude 76 

Influence  of  Altitude 77 

Influence  of  Land  and  Water 77 

Climatic  Zones 78 

Isothermal  Lines 79 

Temperature  Extremes      .        .        . 84 


CHAPTER   VII.     Winds 

Air  Pressure 85 

Measurement  of  Air  Pressure 85 

Change  in  Air  Pressure .         .88 

Planetary  Winds .         .         .90 

Theoretical  Circulation 90 

Trade-Wind  Circulation 91 

Prevailing  Westerlies 93 

Periodical  Winds 95 

Monsoons 95 

Land  and  Sea  Breezes .97 

Mountain  and  Valley  Breezes 98 

Irregular  Winds 99 

Velocity  of  the  Wind 99 

Measurement  of  Winds 101 


^•V  CONTENTS 

„      CHAPTER  VIII.     Storms 

PAsa 

Weather  Changes ^^2 

Weather  Maps *  '     ,^2 

Comparison  of  Weather  Maps .*        *  .104 

Cyclonic  and  Anticyclonic  Areas       •«....'     107 

The  Low-  and  High-Pressure  Areas    ••..!.     107 

Origin  of  the  High- and  Low-Pressure  Areas      .         .         .  .Ill 

Explanation  of  the  Winds *  112 

Explanation  of  the  Rain *  113 

Explanation  of  the  Temperatures II4 

Hurricanes  or  Tropical  Cyclones        .         .         .         .         .         '.  ,     \\q 

Time  and  Place  of  Occurrence 116 

Characteristics 117 

Explanation 118 

Storm  Winds '        '         '  IIQ 

Thunder  Storms °        ]        '  121 

Tornadoes '  '  -lo.. 

124 

CHAPTER   IX.     Moisture  in  the  Atmosphere 

^^P^""         •••••......  126 

Instruments  for  Measuring  Vapor     .  129 

"«'', ::;.■;  130 

*™^' 132 

l^^ 133 

r, :   :  :   :   ;  IS 

Clouds ^gg 

Cloud  Materials '.'*'*  135 

Forms  of  Clouds          •...*.*.'!!'  136 

Causes  of  Clouds        .        .        .        .        ]        *        *        '  1^9 

Rain  ....                                ^\^ 

r-  ••••.::::::::  11: 

^^"^^ 141 

Measurement  of  Rainfall j^ 

Nature  of  Rainfall .'!**'  144 

Distribution  of  Rain \        \        *        *         *  145 

Distribution  of  Snowfall    •...'!.'        .1  143 


CONTENTS  XV 


CHAPTER   X.     Climate 

PA6B 

Meaning  of  the  Word  Climate 149 

Climatic  Zones 149 

Climate  of  the  Tropical  Zone 150 

Belt  of  Calms 150 

The  Trade-Wind  Belt .152 

The  Indian  Climate        .,....-...  154 

Climates  of  the  Frigid  Zones 155 

The  South  Frigid  Zone  . 155 

Near  the  Arctic  Circle ^155 

In  the  Higher  Latitudes 156 

Climates  of  the  Temperate  Zone 160 

Various  Types 160 

United  States  Climates 161 

Difference  between  United  States  and  Europe     ....  162 

Variation  with  Altitude 163 

Differences  between  Ocean  and  Land 163 


CHAPTER  XI.    Distribution  of  Animals  and  Plants 

Zones  of  Life 165 

Life  in  the  Ocean 
Plants 165 


167 
167 
169 
171 


173 


Animals . 

Faunas  of  the  Coast  Line  (Littoral  Faunas) 
Animals  of  the  Ocean  Bottom  (Abyssal  Fauna) 
Life  at  the  Surface  (Pelagic  Faunas)    . 

Life  in  Fresh  Water  . 
Life  on  the  Land 

Plants 174 

Animals         .        .        .        .        c 178 

Distribution  of  Man 180 

Modes  of  Distribution  of  Animals  and  Plants     .        .        o        .        .  182 

Barriers  to  the  Spread  of  Life 186 


xvi  CONTENTS 

Part  III.     The  Ocean 
CHAPTER   XII.     General  Description  of  the  Ocean 

PAQE 

Area  of  the  Ocean 187 

Importance  of  the  Ocean 187 

The  Ocean  Water  is  Salt 188 

Temperature  of  the  Ocean  Surface .  189 

Life  on  the  Bottom 191 

Methods  used  in  Studying  the  Ocean  Bed 193 

Ocean  Bottom  Temperatures 196 

The  Depth  of  the  Sea 198 

Topography  of  the  Ocean  Bottom 200 

The  Ocean  Bed 203 

Globigerina  Ooze 203 

Red  Clay 204 

CHAPTER   XIII.    Tfie  Movements  of  the  Ocean 

Wind  Waves         .        .        .        .       '. 205 

The  Tides 210 

Nature  of  the  Tides 210 

Causes  of  Tides 211 

Effects  of  the  Tides .213 

Ocean  Currents 216 

Differences  in  Temperature 216 

Atlantic  Currents 216 

The  Explanation 218 

Effects 219 


Part  IV.     The  Land 
CHAPTER   XIV.    The  Earth's  Crust 

Condition  of  the  Crust 220 

Minerals  of  the  Crust 221 

Elements 221 

Definition 222 


CONTENTS  xvii 

Minerals  of  the  Crust : 

Quartz 222 

Feldspar 223 

Calcite 224 

Rocks  of  the  Crust 226 

Igneous  Rocks 226 

Sedimentary  Rocks 228 

Metamorphic  Rocks 231 

Position  of  the  Rocks 232 

Movements  of  the  Crust 234 

Age  of  the  Earth 236 

Geological  Ages 238 

CHAPTER   XV.     The  Wearing  Away  of  the  Land 

Entrance  of  Water  into  the  Earth 240 

Return  of  Underground  Water  to  the  Surface 242 

Springs 242 

Artesian  Wells .243 

Mineral  Springs .        .        .  244 

Limestone  Caves 246 

Breaking  Up  of  the  Rocks 248 

Methods  Employed 248 

Difference  in  Result 251 

Effects  of  Weathering  . 254 

Erosion  of  the  Land      .        .        , 257 

Destruction  of  the  Land 260 

CHAPTER  XVL     River  Valleys,  including  Waterfalls 
AND  Lakes 

Characteristics  of  River  Valleys 261 

The  River  Work 264 

History  of  River  Valleys 268 

Accidents  interfering  with  Valley  Development         ....  274 

The  River  Course 278 

River  Deltas 283 

River  Floodplains ■ 286 

Waterfalls • 288 

Lakes ,        .  291 


iVlll  CONTENTS 

CHAPTER  XVII.    Glaciers  and  the  Glacial  Period 

PAGK 

Valley  Glaciers 294 

The  Greenland  Glacier .300 

Icebergs         . 304 

Glacial  Period       .        . 305 

Evidence  of  this 305 

Cause  of  the  Glacial  Period 308 

Glacial  Deposits 309 

Effects  of  the  Glacier 310 

CHAPTER  XV-III.    Sea  and  Lake  Shores 

Difference  "between  Lake  and  Sea  Shores 313 

Form  of  the  Coast 313 

Sea  Cliffs 314 

The  Beach 317 

"Wave-carved  Shores 320 

A  Sinking  Coast 321 

A  Rising  Coast 323 

Marshes 323 

Coral  Reefs 324 

Islands .326 

By  Construction 326 

By  Destruction 328 

Promontories 330 

Changes  in  Coast  Line 331 

CHAPTER  XIX.    Plains,  Plateaus,  and  Mountains 

Plains 332 

Plateaus 334 

Treeless  Plains 335 

Mountains. 

Nature  of  Mountains 335 

Development  of  a  Mountain  System 337 

The  Destruction  of  Mountains 340 

Other  Kinds  of  Mountains 342 

The  Cause  of  IMountains 342 


CONTENTS  XIX 


CHAPTER  XX.     Volcanoes,  Earthquakes,  and  Geysers 

Volcanoes  paqh 

Birth  of  a  Volcano 344 

Vesuvius 345 

Krakatoa 347 

The  Hawaiian  Volcanoes 349 

Other  Volcanoes 351 

Materials  Erupted 351 

Form  of  the  Cone 352 

Extinct  Volcanoes         .        .        .        •• 353 

Distribution  of  Volcanoes 355 

Cause  of  Volcanoes 356 

Explanation  of  the  Differences  in  Volcanoes 356 

Earthquakes 357 

Greysers 361 


ILLUSTRATIONS 


PHOTOGRAPHS  AND  DIAGRAMS 

FIG.  PAGE 

1.  Ocean  surface  showing  curvature  of  earth "3 

2.  Diagram  to  illustrate  curvature  of  earth 4 

3.  Maps  to  illustrate  latitude  and  longitude 5 

4.  Diagrammatic  section  from  surface  to  interior  of  earth    .        .  9 

5.  The  two  hemispheres 10 

6.  Land  and  water  hemispheres 11 

7.  Section  across  South  America,  Atlantic  Ocean,  and  Africa       .  12 

8.  Globe  to  illustrate  day  and  night  at  Equinox     ....  16 

9.  Globe  illustrating  conditions  in  northern  summer      ...  18 

10.  Globe  illustrating  conditions  in  northern  winter        ...  19 

11.  Diagram  to  illustrate  cause  for  seasons 21 

12.  Diagram  illustrating  small  amount  of  heat  reaching  earth        .  22 

13.  Relative  size  and  distance  of  planets  and  sun    ....  24 

14.  Craters  on  the  moon 25 

15.  Diagram  illustrating  Nebular  Hypothesis 29 

16.  Andromeda  Nebula 31 

17.  Diagram  illustrating  density  of  air 40 

18.  Ideal  section  of  atmosphere          .......  63 

19.  Daily  temperature  change,  summer 66 

20.  Thermograph 69 

21.  Thickness  of  air  passed  through  by  vertical  and  oblique  rays    .  70 

22.  Diurnal  variation  of  temperature 70 

23.  Daily  temperature  range  for  several  places         .        .        .        .71 

24.  Influence  of  ocean  on  daily  temperature  range  ....  72 

25.  Daily  range  in  desert  and  humid  tropical  lands  .        .        .        .73 

26.  Irregularities  in  daily  temperature  range 74 

27.  Temperature  range  for  several  days 74 

28.  Seasonal  temperature  range,  several  places         .        .        .        .  -  75 

29.  Seasonal  temperature  range,  several  places        .        ...  76 

xxi 


xxii  ILLUSTRATIONS 

FIG.                                      _  PACK 

30.  Influence  of  ocean  on  seasonal  temperature  range      ...  78 

31.  Isothermal  chart  for  New  England 83 

32.  Diagram  showing  changes  of  pressure 86 

33.  Aneroid  barometer 87 

34.  Diagram  showing  general  air  circulation 90 

35.  Ideal  circulation  of  surface  air,  southern  hemispht  re         .        .  93 
30.  Monsoon  of  Spanish  peninsula 95 

37.  Summer  and  winter  monsoon,  India 96 

38.  Effect  of  sea  breeze  on  daily  temperature  range         ...  98 

39.  Disturbance  of  wind  by  surface  irregularities     ....  99 

40.  Pulsation  of  wind 100 

41.  Anemometer 101 

42.  Weather  conditions,  Jan.  7,  1893        .        .        .        .        .        ,103 

43.  Weather  conditions,  Jan.  8,  1893 104 

44.  Weather  conditions,  Jan.  9,  1893 105 

45.  Paths  followed  by  low-pressure  areas,  November,  1891      .        .  107 

46.  Weather  conditions,  April  20,  1893 108 

47.  Weather  conditions,  Nov.  27,  1890 109 

48.  Weather  conditions,  Jan.  12,  1897 110 

49.  Theoretical  air  movement  in  storm 112 

50.  Theoretical  air  circulation  in  anticyclone 112 

51.  Tropical  cyclone  in  India 116 

52.  Change  in  barometer  in  hurricane 117 

53.  Temperature  change  in  cold  wave  and  sirocco    .        .        .        .  119 

54.  Influence  of  cyclone  and  anticyclone  on  temperature         .        .  120 

55.  Temperature  change  during  chinook,  Montana  ....  121 

56.  Photograph  of  distant  thunder  storm 122 

57.  Map  showing  location  of  thunder  storms  in  cyclonic  area  .        .  123 

58.  Tornado  near  St.  Paul,  Minn 125 

59.  Diagram  showing  change  in  relative  humidity    ....  128 

60.  Psychrometer        .        . 129 

61.  Upper  surface  of  valley  fog 134 

62.  Clouds  on  cliff  in  the  Yosemite 136 

63.  Cumulus  clouds 137 

64.  Cirrus  clouds 138 

65.  Strato-cumulus  clouds 138 

66.  Cirro-cumulus  clouds 139 

67.  Photograph  of  large  hailstones 141 

68.  Photograph  of  snow  flakes 142 

69.  Rain  gauge 143 


ILLUSTRATIONS 


XXlll 


no. 

70.  Desert  vegetation  in  the  west     . 

71.  Midnight  sun,  northern  Norway 

72.  Ice-covered  sea  in  the  Arctic 

73.  Land  in  Greenland,  summer 

74.  Greenland  ice  sheet     .... 

75.  Cold  wave,  March  13,  1888 

76.  Map  showing  snowfall  of  United  States 

77.  Mangrove  swamp,  Bermuda 

78.  Seaweed  mat.  Cape  Ann,  Mass. 

79.  Corals,  Great  Barrier  reef,  Australia 

80.  Deep-sea  fish 

8 1 .  Deep-sea  crinoid  .... 

82.  Semi-tropical  forest  in  Florida   . 

83.  Cactus  in  Arizona  desert    . 

84.  Mountain  peak,  crest  of  Andes,  Peru 

85.  Near  timber  line.  Rocky  Mountains  . 

86.  Arctic  flora  in  snow  '. 

87.  A  Bermuda  road         .... 

88.  Bit  of  Bermuda  landscape  . 

89.  Arctic  sea  ice 

90.  Deep-sea  sounding  machine 

91.  Deep-sea  dredge  on  ocean  bottom 

92.  Temperature  of  ocean  bottom  of  north  Atlantic 

93.  Temperature  of  ocean  at  various  depths 

94.  Section  from  Atlantic  to  Gulf  of  Mexico 

95.  Section  of  ocean  from  New  York  to  Bermuda 

96.  Section  across  Atlantic  showing  temperature  and 

97.  Ocean  bottom  topography  (Jones  model) 

98.  Ocean  bottom  topography  (Jones  model) 

99.  Diagram  showing  approach  of  wave  on  beach 

100.  Diagram  illustrating  origin  of  tidal  wave  . 

101.  Diagram  showing  advance  of  tidal  wave  in  Atlantic 

102.  Diagram  showing  cause  of  spring  and  neap  tides 

103.  Diagram  showing  currents  of  eastern  north  Atlantic 

104.  Stratification  in  horizontal  rocks 

105.  Quartz  crystal 

106.  Piece  of  calcite 

107.  Section  of  diabase  enlarged  by  microscope 

108.  Diagram  illustrating  intrusion  of  granite  . 

109.  Consolidated  pebble  bed     .... 


depth 


PAGE 

153 
156 
157 
158 
150 
160 
161 
166 
167 
169 
170 
171 
174 
175 
170 
177 
178 
182 
183 
190 
192 
194 
195 
190 
197 
199 
200 
201 
202 
207 
210 
211 
212 
217 
220 
222 
224 
226 
227 
229 


xxiv  ILLUSTRATIONS 

FIG,  _  PAGE 

110.  Beach,  Cape  Ann,  Mass 230 

111.  Coquina 231 

112.  Diagram  of  volcano  in  cross  section 233 

113.  Crumpling  of  rock 233 

114.  Diagram  illustrating  faults 235 

115.  Photograph  of  small  fault 236 

116.  Folded  rocks 236 

117.  A  monocline 237 

118.  Section  of  gneiss  enlarged  by  microscope 241 

119.  Diagram  illustrating  conditions  in  hot  springs  ....  242 

120.  Diagram  illustrating  cause  of  hillside  springs    ....  242 

121.  Diagram  illustrating  artesian  wells 243 

122.  Diagram  illustrating  artesian  wells 244 

123.  Hot  Springs,  Yellowstone .        .245 

124.  Howe's  Cave,  New  York 246 

125.  Natural  Bridge 247 

126.  Column  in  cavern 248 

127.  Effect  of  frost  action  on  mountain  top 249 

128.  Effect  of  weathering  in  arid  lands 252 

129.  Butte  in  western  Texas      .        . 253 

130.  Crumbling  of  rocks  on  mountain  side 266 

131.  Decaying  granite,  Maryland 256 

132.  Diagram  illustrating  residual  soil       .        .        .        .        .        .  257 

133.  Bad  Lands,  South  Dakota .268 

134.  River  gorge  in  Peruvian  Andes 262 

136.     Rocky  stream  bed  in  Adirondacks 264 

136.  Enfield  Gorge,  Ithaca,  N.Y 265 

137.  Diagram  illustrating  stream  action  and  weathering  .        .        .  267 

138.  Meandering  of  Missouri  River 268 

139.  Young  valley,  central  New  York 269 

140.  Diagram  illustrating  base  level 270 

141.  Diagram  illustrating  development  of  stream  valley  .        .        .  271 

142.  Broad  mature  valley,  Ithaca,  N.Y 271 

143.  Delaware  Water  Gap  ........  272 

144.  Cross  section  of  Colorado  River 274 

145.  View  in  Colorado  cailon 274 

146.  Diagram  illustrating  Chesapeake  Bay  river  system  .        .        .  276 

147.  Drainage  in  mountain 279 

148.  Drainage  on  plain 279 

149.  Interlocking  tributaries 280 


ILLUSTBATIONS  XXV 

FIG.  PAGE 

150.  Changing  of  mountain  tops  to  valleys 281 

151.  Elver  flowing  in  anticline 282 

152.  Cross  section  of  delta 283 

153.  Alluvial  fans  in  the  west    .        . 285 

154.  Waterfall,  central  New  York 288 

155.  General  view  of  Niagara  Falls 289 

156.  Peat  bogs  in  Adirondacks 290 

157.  Snowfield  in  high  Alps 295 

158.  An  Alpine  glacier 296 

159.  Crevassed  surface  of  Muir  glacier,  Alaska        ....  297 

160.  Margin  of  Cornell  glacier,  Greenland         .        .        .        .        ,  301 

161.  Delta  in  glacier  lake 302 

162.  Scratched  glacial  pebble,  Greenland  ......  302 

163.  Ice  floating  in  water 303 

164.  Iceberg  off  North  Greenland  coast 304 

165.  Map  of  United  States  showing  extension  of  ice  in  glacial  period  305 

166.  Boulder  clay,  Cape  Ann,  Mass 306 

167.  Glaciated  rock  surface  in  Iowa 307 

168.  Terminal  moraine  hills,  Ithaca,  N.Y 308 

169.  Boulder-strewn  moraine.  Cape  Ann,  Mass 309 

170.  Sea  cliff,  Bermuda 315 

171.  Undercut  sea  cliffs,  Bermuda 316 

172.  Wave-cut  cliff.  Lake  Superior 318 

173.  Bar  across  bay.  Cape  Breton,  Nova  Scotia        ....  319 

174.  Bars  on  shore  of  Martha's  Vineyard 320 

175.  Tiny  wave-carved  bay.  Cape  Ann,  Mass 321 

176.  Depressed  coast  of  part  of  Connecticut 322 

177.  Beach  and  coral  reef,  coast  of  Florida 325 

178.  Serpula  atolls,  Bermuda     .        .        .        .        .        .        .        .  327 

179.  Wave-cut  islands,  shore  of  Bermuda 328 

180.  Islands  caused  by  sinking  of  Bermuda 329 

181.  Island  joined  to  land  by  bars 330 

182.  Plain  of  Everglades,  southern  Florida       .        .        .        .        .  332 

183.  Diagram  illustrating  dissection  of  plain 334 

184.  Sections  of  Appalachian  Mountains 336 

185.  Grandfather  Mountain,  North  Carolina 337 

186.  Mount  Moran,  Teton  Mountains 339 

187.  Near  timber  line,  Gallatin  Mountains,  Montana       .        .        .  341 

188.  Pompeii  and  Vesuvius ^,  .  347 

189.  Mauna  Loa,  Hawaiian  Islands  .......  348 


XXVI 


ILLUSTRATIONS 


FIQ.  PASS 

190.  Crater  of  Kilauea 349 

191.  Tiny  volcano,  Mediterranean 350 

192.  Popocatapetl,  Mexico 353 

193.  Volcanic  necks,  New  Mexico 354 

194.  Distribution  of  Volcanoes 355 

195.  Effect  of  Japanese  earthquake,  1891 358 

196.  Diagram  illustrating  earthquake  wave 359 

197.  Isoseismals,  Charleston  earthquake 360 

198.  Giant  Geyser,  Yellowstone 361 


PLATES 


FACING  PAGB 


Granite  peaks  in  the  Yosemite     . 

1.  Isothermal  chart  of  the  world  for  July 

2.  Isothermal  chart  of  the  world  for  January  . 

3.  Isothermal  chart  of  the  world  for  the  year . 

4.  Isothermal  chart  of  the  United  States  for  July 

5.  Isothermal  chart  of  the  United  States  for  January 

6.  Isothermal  chart  of  the  United  States  for  the  year 

7.  Chart  showing  isobaric  lines  for  the  world  . 

8.  Map  showing  prevailing  winds  of  the  globe  for  July 

9.  Map  showing  prevailing  winds  of  the  globe  for  January 

10.  A  West  Indian  hurricane 

11.  Typical  winter  storm 

12.  Rainfall  chart  of  world 

13.  Rainfall  chart  of  United  States    .... 

14.  Average  temperature  of  sea  surface     . 

15.  Depth  of  Atlantic  Ocean 

16.  Chart  of  ocean  currents 

17.  Three  rock  specimens  (diabase,  granite,  and  gneiss) 

18.  Delta  of  Mississippi 

19.  Map  of  drowned  coast,  Maine     .        .  *      . 

20.  Mountain  ridge  in  the  northwest 


Frontispiece 


79 

80 

81 

82 

83 

84 

90 

92 

93 

116 

117 

146 

147 

189 

198 

214 

227 

283 

314 


ILLUSTRATIONS  XXVll 


ACKNOWLEDGMENT   OF  ILLUSTRATIONS 

Aside  from  those  that  are  origmal,  illustrations  in  this  book  have  been 
obtained  from  the  following  sources.  Some  of  these  have  been  more  or 
less  modified  to  suit  the  needs  of  the  book.  A  very  few  that  have  been 
borrowed  are  not  acknowledged  because  the  original  source  is  not  known. 
I  am  also  indebted  to  Mr.  B,  F.  White  and  Mr.  J.  0.  Martin  for  some  of 
the  photographs. 

Abbe,  Annual  Report,  Signal  Service,  Part  2,  1889,  Figs.  18  and  39. 

Agassiz,  Three  Cruises  of  the  Blake,  Figs.  80,  81,  92,  93. 

Bailey,  Prof.  L.  H.  (Photographs  by),  82,  177,  182. 

Ball,  Popular  Astronomy,  Fig.  11. 

Blanford,  Climates  and  Weather  of  India,  etc. ,  Figs.  37  and  51. 

Buchan,  Challenger  Reports,  Atmospheric  Circulation,  Plates  1,  2,  3,  7, 

8,  and  9. 
Chamberlin,  Third  Annual  Report,  U.  S.  G.  S.,  Fig.  165. 
Button,  Sixth  Annual  Report,  U.  S.  G.  S.,  Fig.  193;  same,  Ninth  Annual 

Report,  Fig.  197. 
Davis,  Series  of  Lantern  Slides  for  Schools,  Fig.  143. 
Freiz,  J.  P.  (Dealer  in  Meteorological  Instruments),  Baltimore,  Md., 

Figs.  20,  33,  41,  60,  69. 
Gardner,  J.  L.,  2d  (Photographs  by).  Figs.  166  and  169. 
Gilbert,   Second  Annual  Report,  U.    S.   G.    S.,  Fig.    153;   same,   Fifth 

Annual  Report,  Fig.  172. 
Hann,  Berghaus  Atlas  der  Meteorologie,  Plate  12,  modified. 
Harvard  College  Astronomical  Observatory  Annals,  Vol.  XXXI.,  Fig.  31. 
Hayden,  West  Indian  Hurricanes,  etc. ,  Fig.  75. 

Haynes,  F.  Jay  (Photographer),  St.  Paul,  Minn.,  Figs.  123,  159,  and  198. 
Hellmann,  Schneekrystalle,  Fig.  68. 
Howes,  C.  H.  (Photographer),  Ithaca,  N.Y.,  Fig.  139. 
Jackson  Photograph  Co.,  Denver,  Col.,  Figs.  62,  85,  126,  145,  155,  186, 

and  192. 
Johnston-Lavis,  South  Italian  Volcanoes,  Fig.  191. 
Jones,  Thomas,  Chicago,  111.,  Photograph  of  copyrighted  globe,  Figs.  97 

and  98. 
Kent,  Great  Barrier  Reef,  Fig.  79. 
Keyes,  Fifteenth  Annual  Report,  U.  S.  G.  S.,  Fig.  131,  Vol.  III.;  Iowa 

Geological  Survey,  Fig.  167. 


XX  viii  ILL  USTRA  TIONS 

Koester   (Photographs   by,   sold   by  Fredrick  and    Koester,   St.    Paul, 

Minn.).     Printed  in  Am.  Met.  Jour.,  VII.,  1891,  Fig.  58. 
Langley,  American  Journal  Science,  Vol.  XLVII.,  1890,  Fig.  40. 
Libbey,  Prof.  W.,  Jr.  (Photographs  by).  Figs.  86,  189,  and  190. 
McGillivray  (Photographer),  Ithaca,  N.Y.,  Fig.  13G. 
Murray,  Challenger  Reports,  Final  Summary,  Plates  14  and  15. 
Nasmyth  and  Carpenter,  The  Moon,  Fig.  14. 
New  York  State  Weather  Bureau  (from  records  of),  Figs.  19,  22,  26,  27, 

32,  52,  53,  54,  and  59. 
Notnian  (Photographer),  Montreal,  Canada,  Plate  20. 
Sigsbee,  Deep  Sea  Sounding  and  Dredging,  Fig.  90. 
Steeruwitz,  First  Annual  Report,  Texas  Geological  Survey,  Fig.  129. 
Stoddard,  S.  R.   (Photographer),  Glens  Falls,  N.Y.,  Figs.  124,  135,  and 

156. 
Thomson,  Challenger  Reports  (Narrative),  Fig.  91.  • 
Thornton,  Advanced  Physiography  (from  a  Photograph  by  Mr.  Roberts), 

Fig.  16. 
United  States  Coast  Survey  (Maps  of),  Plates  18  and  19,  modified. 
United  States  Geological  Survey  (Maps  of).  Figs.   138,  147,  174,  and 

176,  modified. 
United  Spates  Geological  Survey  (Photographs  by),  Figs.  70,  113,  125, 

185,  and  187,  and  Frontispiece. 
United  States  Geological  Survey  Folios  (Campbell),  Fig.  184  (Hayes), 

Fig.  151. 
United  States  Weather  Bureau  (based  upon  maps  and  records  of).  Plates 

4,  5,  6,  10,  11,  and  13,  and  Figs.  42,  43,  44,  45,  46,  47,  48,  55,  57,  and 

76. 
Ward,  Set  of  Cloud  Slides  (Riggenbach,  Burnham,  etc.),  Figs.  56,  61, 

63,  64,  65,  and  66. 
Williston,  Prof.  S.  W.,  Lawrence,  Kansas  (Photographs  by),  Figs.  128 

and  133. 


Part  I 
INTRODUCTION 


PIRST  BOOK  OF  PHYSICAL  GEOGEAPHY 


^>^c 


CHAPTER   I 


CONDITION   OF   THE   EARTH 

Form  of  the  Earth:  The  Earth  a  AS^^erd.-^- Stand (hg 
upon  the  seashore,  and  looking  out  upon  the  broad  ex- 
panse of  water,  we 
see  ships  sailing 
along,  some  near 
at  hand  and  some 
far  away.  Those 
that  are  near  show 
the  sails,  spars,  and 
even  the  hull  down 
to  the  water's  edge, 
but  only  the  sails  and  masts  of  the  more  distant  ones  are 
seen,  and  perchance  one  in  the  offing  is  detected  only  by 
its  topmast  (Fig.  1). 

This  is  because  the  surface  of  the  earth,  and  the  water 
upon  it,  is  curved  (Fig.  2).  The  vessel  gradually  disap- 
pears behind  the  curvature  of  the  earth,  just  as  a  man 
disappears  from  sight  as  he  passes  over  the  crest  of  a 
hill.     There  are  other  proofs  that  the  earth  is  a  spherical 


Fig.  1. 
The  ocean  surface  to  show  curvature  of  the  earth. 


4  FIRST  BOOK  OF  PHYSICAL   GEOGUAPUT 

body.     For  instance,  •  if  we  should  start  on  one  of  the 

ships,  we  might  pass  entirely  around  the  globe  and  return 

to  the  point  whence  we  started.    Or,  by  travelling  over  land 

and  water,  we  can 

A  B 

C^ —  ■ ^__p      go  due  east  or  due 

Fig.  2.  west,   and   in   time 

To  illustrate  curvature  of  earth.    A  person  stand-      fjj^jj  OUrselveS   back 
Ing  at  B  could  not  see  an  object  at  C  unless  it  ""     .  , 

rose  to  the  level  of  the  line  AB.  at  the  starting  point 

(see  a  globe). 

Longitude.  —  Should  a  dozen  people  start  from  as  many 
places,  such  9,8;  New  York,  San  Francisco,  London,  etc., 
and  be.'ablc:tv?  go  due  north,  their  paths  would  all  con- 
verge, toward  .-ja 'point  at  which  they  might  eventually 
-iii-eet-;' and- this 'point,  which  is  so  enwrapped  in  ice  and 
snow  that  it  has  not  yet  been  visited  by  man,  is  called 
the  North  Pole.  Passing  due  south  from  these  same  places, 
the  travellers  would  in  time  meet  at  a  point  in  the  south, 
which  is  called  the  South  Pole,  and  this  region  also  is 
inaccessible  to  man.^ 

In  mapping  the  globe,  geographers  are  in  the  habit  of 
projecting  lines  in  the  direction  of  these  imaginary  jour- 
neys, and  these  all  converge  toward  the  poles.  These 
meridians,  or  lines  of  longitude,  are  360  in  •  number,  and 
each    of    them    is    marked    as    a   degree    of    longitude.^ 

1  The  North  and  South  Poles  are  the  imaginary  points  on  the  surface 
of  the  earth  through  which  the  axis  of  the  earth  emerges.  This  axis  is 
that  about  which  the  earth  rotates,  just  as  a  globe  or  an  apple  may  be 
made  to  rotate  about  an  axis.  These  are  not  the  same  as  the  magnetic 
poles  toward  which  the  compass  needle  points.  The  north  magnetic  pole 
lies  to  the  southvmrd  of  the  true  North  Pole,  and  is  situated  in  Boothia 
Land,  north  of  Hudson's  Bay. 

2  Since  there  are  360  lines  of  longitude  it  follows  that  at  the  equator 
the  length  of  a  degree  of  longitude  is  very  nearly  69.16  miles.      As 


CONDITION  OF  THE  EABTB  5 

The  distance  between  these  varies,  for  they  broaden  out 
and  spread  apart  as  the  distance  from  the  poles  increases 
(Fig.  3).  Where  furthest  apart  the  distance  between  two 
meridians  is  about  69  miles.  Each  degree  (°)  is  divided 
into  60  minutes  ('),  and  each  minute  into  60  seconds  ("), 
and  the  longitude  of  any  place  is  given  in  degrees,  min- 
utes, and  seconds.  Greenwich  Observatory,  England,  has 
been  chosen  by  English-speaking  people  as  the  place  from 


Fig.  3. 
To  illustrate  latitude  and  longitude. 

which  to  start  in  numbering  these  degrees.  ^  From  Green- 
wich as  the  zero,  the  meridians  are  numbered  toward  the 
east  until  180°  is  reached,  and  this  is  known  as  east  lon- 
gitude^ while  west  longitude  is  located  in  the  same  way 


we  proceed  toward  the  poles  the  length  of  a  degree  of  longitude  becomes 
less  and  less  until  it  is  0  at  the  poles. 

1  In  France  the  Observatory  of  Paris  is  taken  as  the  starting  place,  and 
this  lies  2°  20'  9"  east  of  Greenwich  j  but  English-speaking  people  do  not 
use  this. 


6  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

toward  the  west.  Thus  the  United  States  is  in  west 
longitude. 

Latitude.  —  In  order  to  locate  places  on  a  sphere,  we 
must  know  not  only  the  longitude,  or  the  east  or  west 
distance  from  a  place,  but  also  the  distance  in  a  north 
or  south  direction  from  some  definite  part  of  the  earth. 
Therefore  a  series  of  imaginary  circles  are  passed  around 
the  earth  in  an  east  and  west  direction.  In  numbering 
these,  the  zero  circle  is  placed  midway  between  the  two 
poles,  and  to  this  the  name  Equator  is  applied.  The  space 
between  each  pole  and  the  Equator  is  divided  into  90°, 
the  length  of  a  degree  of  latitude  varying  somewhat,  but 
being  about  69  miles.  These  degrees  are  also  divided  into 
minutes  and  seconds. 

Since  degrees  of  latitude  are  numbered  from  the  Equa- 
tor as  zero,  toward  each  pole,  high  latitudes  are  nearer  the 
poles  and  low  latitudes  near  the  Equator.  All  north  of  the 
Equator  is  called  the  northern  hemisphere^  and  all  south 
of  it  the  southern  hemisphere.  Since  latitude  is  measured 
both  north  and  south  of  the  Equator,  there  is  noi^th  lati- 
tude and  south  latitude^  the  United  States  being  in  the 
former.  Since  we  know  the  size  of  the  globe,  if  we  deter- 
mine the  latitude  and  longitude  of  any  given  place,  we 
can  easily  tell  its  exact  distance  from  any  other  known 
part  of  the  earth. ^     / 

The  Earth  an  Oblate  Spheroid.  —  The  diameters  of  a  true 
sphere  must  be  the  same  in  all  parts;  but  that  of  the 
earth  is  7899.1  miles,  measured  along  the  axis  from  pole 
to  pole,  and  7925.6  miles  at  the  Equator.     This  shows  a 

1  It  would  be  well  to  spend  some  time  upon  this  subject,  giving  the 
students  some  practice  lessons,  so  that  they  may  fully  grasp  the  meaning 
of  latitude  and  longitude. 


CONDITION  OF  THE  EARTH  7 

slight  flattening  in  the  polar  regions,  and  a  protuberance, 
or  bulging,  in  the  equatorial  part,  and  the  sphere  is  thus 
distorted  into  an  oblate  spheroid.  In  an  ordinary  study 
of  the  earth's  surface  this  flattening  by  about  13^  miles  at 
each  pole  would  not  be  noticed ;  but  in  the  movement  of 
the  earth  through  space,  this  deflection  from  a  sphere  has 
produced  very  marked  effects.  Because  of  this  difference, 
the  length  of  the  degree  of  latitude  varies  from  equatorial 
to  polar  regions,  being  less  than  68  miles  in  India,  and  a 
little  more  than  69  miles  in  Sweden. 

General  Condition  of  the  Earth.  — Speaking  broadly,  there 
are  three  parts  to  the  earth :  (1)  the  solid  earth  itself ; 
(2)  the  partial  water  envelope ;  and  (3)  the  gaseous 
envelope,  or  atmosphere. 

Air.  —  The  air  is  in  constant  movement,  performing 
many  tasks  of  importance.  We  breathe  it ;  it  gives  life 
to  plants  and  animals;  it  diffuses  the  heat  and  light  which 
reach  the  earth  from  the  sun;  it  brings  us  our  winds, 
clouds,  and  storms ;  it  furnishes  the  oxygen  by  which  our 
lamps  may  burn  and  our  fires  glow;  it  ruffles  the  ocean 
surface  with  waves,  and  drives  our  ships  along ;  and  in 
many  hundred  other  ways  it  serves  us.  Yet  the  air  is 
merely  a  thin,  transparent  mass  of  gas,  whose  constant 
presence  about  us  is  hardly  realized  (Part  II). 

Ocean. —  The  ocean  shuts  out  from  view  nearly  three- 
fourths  of  the  solid  earth,  and  in  places  buries  it  beneath 
a  depth  of  four  or  five  miles  of  water.  Its  surface  is  so 
nearly  level  (that  is  to  say,  it  is  parallel  to  the  general 
surface  of  the  globe),  that  we  may  sail  upon  it  for  thou- 
sands of  miles  without  a  glimpse  of  any  other  irregularity 
than  the  waves  which  disturb  its  surface. 

Like  the  air,  the  ocean  enwraps  the  globe,  and  conforms 


8  FIBST  BOOK  OF  PHTStCAL  GEOGRAPHY 

to  its  general  outline,  being  held  in  place  by  the  force  of 
gravity,  by  which  the  earth  binds  to  itself  all  movable 
objects  on  its  surface.  This  level  water  surface,  the  sea- 
level^  is  the  plane  from  which  we  determine  the  elevations 
on  the  land.  It  is  not  strictly  level,  but  is  slightly  dis- 
torted by  various  causes. 

The  Solid  Earth,  —  Some  portions  of  the  land  are  still 
inaccessible,  and  great  areas  near  each  pole  have  so  far 
baffled  all  the  efforts  of  venturesome  explorers;  but  while 
we  have  now  visited  most  lands,  our  knowledge  almost 
ceases  when  we  pass  below  the  very  surface.  Accumu- 
lated on  the  surface  there  is  generally  a  soil,  and  beneath 
this,  usually  at  depths  of  only  a  few  feet,  hard  rock  of 
various  kinds  is  encountered.  Here  and  there  wells  and 
mines  pierce  to  the  depth  of  a  mile  or  more,  and  to  this 
depth,  at  least,  the  solid  rock  extends;  but  we  can  only 
speculate  concerning  the  conditions  below  this  level. 

In  all  deep  borings  and  shafts,  it  is  found  that  the  tem- 
perature of  the  earth  increases  with  the  depth ;  and  while 
there  is  a  considerable  variation  from  place  to  place,  the 
average  condition  shows  an  increase  of  about  1°  for  every 
50  or  60  feet  of  descent.  If  this  continues  toward  the 
centre,  as  it  probably  does,  the  temperature  of  the  earth 
must  be  very  high  at  the  depth  of  a  few  score  of  miles. 
Indeed,  it  seems  that  at  great  depths  the  temperature 
must  be  higher  than  the  melting  point  of  rocks  at  the 
surface.  In  fact,  here  and  there  'melted  rock  comes  to  the 
air,  through  cracks  reaching  down  into  the  earth,  and  in 
this  case  volcanoes  are  formed. 

It  was  once  believed  that  these  facts  proved  the  earth  to 
be  a  great  globe  of  liquid,  molten  rock,  around  which  was 
a  solid  rind  or  crust     But  astronomers  have  shown  that 


CONDITION  OF  THE  EARTH 


9 


A  T       O      [ 


this  cannot  be ;  for  in  its  behavior  toward  the  planets,  the 
earth  acts  like  a  rigid  bodi/,  and  if  there  is  molten  mate- 
rial, the  outer  crust  must  be 
very  thick.  Scientists  now 
believe  that  the  interior  is 
highly  heated^  but  that  it  is 
kept  in  a  solid  condition  by 
the  great  weight  of  the 
overlying  rock.^  We  still 
use  the  term  earth's  crust  as 
a  convenient  word  to  express 
the  solid  and  relatively  cold 
outer  part  of  the  earth. 

Surface  of  the  Earth  :  Con- 
tinents mid  Ocean  Basins.  — 
Wlien  the  earth  is  repre- 
sented by  a  map  or  globe,  it 
is  customary  to  make  the 
surface  perfectly  smooth ; 
yet  we  all  know  that  the 
earth's  surface  is  very  irreg- 
ular. This  is  because  the 
irregularities  with  which  we 
are  familiar  are  small  com- 
pared to  the  size  of  the  earth. 
While  the  diameter  of  the 
sphere  is  about  7900  miles, 
the  greatest  irregularity  of 
the  land  above  sea-level  is 
only-  about  five  miles. 

1  It  may  be  stated  that  an  increase  of  pressure  raises  the  melting  point ; 
and  at  a  depth  of  several  miles  in  the  earth,  the  pressure  of  the  load  of 


Fig.  4. 
Section  to  show  relative  amount  of  air 
and  solid  earth,  and  the  supposed 
condition  within  the  earth. 


10 


FIRST  BOOK  OF  PHYSICAL  GEOGBAPHT 


The  surface  is  diversified  by  a  series  of  grand  elevations 
and  depressions,  the  full  extent  of  which  is  obscured  by 
the  ocean.  The  continents  are  the  elevations,  the  ocean 
beds  the  depressions.  There  are  two  sets  of  continents, 
the  New  World,  including  North  and  South  America,  and 
the  Old  World,  including  Eurasia,  Africa,  and  Australia. 


NORTHERN  HEMISPHERE 


SOUTHERN  HEMISPHERE 


Fig.  5. 
The  two  hemispheres,  showing  the  grouping  of  continents  and  oceans. 

On  a  rather  arbitrary  basis  it  is  customary  to  divide  these 
land  masses  into  individual  continents.  The  smallest, 
Australia,  is  quite  completely  separated  from  the  others, 
though  there  is  a  partial  connection  with  Asia  by  way  of 
the  East  Indies.  Europe  and  Asia  cannot  be  naturally 
separated.  Africa  is  removed  from  Eurasia  only  by  the 
relatively  narrow  Mediterranean  and  Red  seas,  being  con- 
nected at  the  Isthmus  of  Suez ;  and  the  two  Americas  are 
directly  connected  by  the  Isthmus  of  Panama,  and  partly 
also  by  the  West  Indies  and  the  Antilles. 


rock  above  must  be  very  great  • 
impossible. 


so  great,  in  fact,  that  melting  may  be 


CONDITION   OF  THE  EARTH 


11 


Between  these  groups  of  continents  there  are  two  great 
oceans,  the  Atlantic  and  Pacific,  while  between  the  Afri- 
can and  Australian  prolongation  of  the  Old  World  land- 
group,  is  another  large  ocean,  the  Indian.  Around  each 
pole  there  is  some  land  and  much  water.  That  around 
the  South  Pole  is  called  the  Antarctic  Ocean,  and  that  sur- 
rounding the  North  Pole  the  Arctic.  Although  open  to  the 
Atlantic,  the  Arctic  is  much  more  enclosed  than  the  Ant- 
arctic, which  has  no  natural  boundary  line  between  either 
the  Pacific,  Indian,  or  Atlantic.     The  Arctic  may  be  con- 


pHEM.SP^,g^ 


^epHEMISP/y 


Fig.  6. 
Land  and  water  hemispheres. 


sidered  to  be  a  northern  prolongation  of  the  Atlantic,  and 
the  Antarctic  to  be  parts  of  the  Pacific,  Atlantic,  and 
Indian. 

Viewing  the  globe  as  a  whole,  we  find  that  the  water  predominates 
in  the  southern  hemisphere,  and  the  land  in  the  northern.  It  is  also 
noticeable  that  the  water  projects,  in  somewhat  triangular  tongues, 
from  the  great  nucleus  around  the  South  Pole,  toward  the  North 
Pole,  and  that  the  continent  groups  project  somewhat  triangular 
tongues  from  the  land  area  of  the  northern  hemisphere  toward  the 
South  Pole.     This  development  of  the  land  in  one  hemisphere,  and 


12  FIRST  BOOK  OF  PHYSICAL  GEOGRAPnV 

the  water  in  the  other,  makes  it  possible  to  divide  the  earth  into  two 
hemispheres,  in  one  of  which  there  is  little  land,  while  in  the  other 
the  land  is  distinctly  in  excess  of  the  water  (Fig.  6).  These  are 
called  the  land  and  water  hemispheres. 

Not  only  does  the  sea  cover  a  greater  area  than  the 
laiid,^  but  the  average  elevation  of  the  land  is  much  less 
than  the  average  depth  of  the  ocean.^ 


SOUTH  AMERICA 


M 


Fig.  7. 

Section  across  South  America,  Atlantic  Ocean,  and  Africa,  showing 
greater  depth  of  ocean. 

31ountain  Irregularities.  —  The  second  group  of  irregu- 
larities are  those  occurring  along  relatively  narrow  lines. 
Upon  the  continents,  ranges  and  ridges  of  mountains  rise 
above  the  general  level  of  the  land,  usually  from  the  crest 
of  a  high  plateau.  The  most  remarkable  of  these  moun- 
tain groups  is  that  facing  the  Pacific  in  the  two  Americas, 
and  extending  from  Alaska  to  Cape  Horn.  Occasional 
peaks  in  these  mountains  attain  an  elevation  of  three  or 
four  miles  above  sea-level;  and  often,  as  in  the  case  of  the 

1  The  area  of  the  earth  is  not  far  from  190,700,000  square  miles,  of 
which  about  144,700,000  is  water  surface  and  52,000,000  land,  the  area  of 
the  water  being  about  three-fourths  of  the  total. 

2  The  average  depth  of  the  ocean  is  computed  to  be  about  12,000  feet, 
while  the  average  elevation  of  the  land  above  the  sea,  is  only  about  2500 
feet,  though  if  the  ocean  could  be  removed,  the  continents  would  stand 
as  great  elevations  rising,  on  an  average,  fully  14,000  feet  above  the 
ocean  beds.  In  some  places  of  exceptional  ocean  depth  and  land  height, 
the  difference  between  ocean  bottom  and  mountain  peak  would  amount 
to  about  60,000  feet,  or  over  eleven  miles,  and  in  a  single  view  there  would 
be  some  cases  of  elevation  amounting  to  fully  eight  miles. 


CONDITION  OF  THE  EARTH  13 

Rocky  Mountains,  the  plateau   above  which  they  rise  is 
fully  a  mile  above  the  sea. 

At  times  mountains  extend  into  the  ocean,  as  in  the  case 
of  the  Kamtchatka  peninsula.  By  means  of  these  moun- 
tains, great  peninsulas  and  chains  of  islands,  such  as  the 
Japanese  group,  partially  cut  off  and  enclose  arms  of  the 
sea.  Very  often,  elevations  rise  entirely  in  the  sea,  per- 
haps in  mid-ocean,  and  then,  as  in  the  Hawaiian  Archi- 
pelago, there  are  produced  chains  of  oceanic  islands.  These 
are  often  the  higher  peaks  of  a  partly  submerged  mountain 
range ;  and  not  uncommonly  they  are  volcanic  cones,  just 
as  some  of  the  higher  peaks  of  the  mountains  on  the  land 
are  volcanoes  (Chapter  XX). 

Minor  Irregularities.  —  There  are  many  minor  irregularities  of  the 
land,  wliich  are  mainly  the  result  of  the  carving  of  the  surface,  by  the 
weather,  rivers,  and  ocean.  By  these  forces  the  land  is  constantly 
being  cut  into  hills  and  valleys,  so  that  in  the  course  of  long  periods 
of  time,  our  land  surface  has  become  very  much  worn,  dissected,  and 
sculptured.  Some  of  the  causes  for  these  irregularities  are  described 
in  the  chapters  on  the  land  (Part  IV). 

Movements  of  the  Earth :  Rotation.  —  Every  day,  over 
most  of  the  earth,  the  sun  rises  in  the  eastern  sky,  and  after 
travelling  across  the  heavens,  sets  in  the  west.  Between 
sunrise  and  sunset  the  earth  is  bathed  in  light  and  heat; 
at  night  darkness  prevails,  and  coolness  takes  the  place  of 
warmth.  Before  it  was  known  that  the  earth  was  a  sphere, 
it  was  thought  that  the  sun  actually  rose  and  set,  making 
a  daily  journey  across  the  heavens  ;  but  for  a  long  time  we 
have  known  that  this  apparent  movement  of  the  sun  is 
really  due  to  the  motion  of  the  earth.  Our  globe  is  spin- 
ning about  an  axis  which  passes  through  the  poles,  as  one 
might  make  an  apple  rotate  by  using  the  stem  as  an  axis. 


14  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

This  spinning  or  rotation  of  the  earth  is  constant  and 
quite  uniform,  and  the  complete  rotation  is  made  in  a 
little  less  than  24  hours  (23  hours  and  5Q  minutes),  cfr  the 
time  between  two  sunrises.  So,  as  the  earth  rotates,  turn- 
ing toward  the  east,  the  sun  appears  to  rise  in  the  east  and 
to  move  across  the  heavens  as  the  day  advances.  If  we 
could  travel  across  the  earth  at  the  Equator,  going  at  the 
rate  of  about  69  miles  in  4  minutes,  the  position  of  the 
sun  in  the  heavens  would  remain  the  same :  starting  at 
the  sunrise,  the  sun  would  remain  on  the  eastern  horizon, 
and  at  the  end  of  the  24  hours  we  would  be  at  the  starting 
place,  with  sunrise  still  present;  but  those  who  remained 
behind  would  have  experienced  the  complete  changes  of 
day  and  night. 

This  is  the  same  as  saying  that  the  earth  rotates  at  this 
rate,  and  that  the  sun's  rays  advance  over  the  land  in 
this  rapid  way.  But  the  diameter  of  a  circle  of  latitude 
decreases  from  the  Equator  toward  the  poles,  and  there- 
fore the  sun's  rays  creep  across  the  globe  at  a  less  and  less 
rapid  rate  as  the  distance  from  the  Equator  increases.  It 
takes  the  earth  about  24  hours  to  make  the  complete  rota- 
tion, whether  at  the  Equator  or  near  the  poles ;  and  hence 
the  time  between  two  sunrises  is  everywhere  the  same, 
although  the  distance  over  which  the  rays  pass  from  hour 
to  hour  decreases  toward  the  poles. 

The  division  of  the  earth  by  meridians  is  based  on  this 
fact,  the  distance  between  two  of  these  lines  being  that 
which  the  sun  passes  in  about  4  minutes.  This  distance 
is  greatest  at  the  Equator,  and  hence  the  meridians  spread 
further  apart  as  the  equatorial  belt  is  approached.  The 
sun  travels  from  meridian  to  meridian  in  4  minutes ;  and 
as  there  are  24  hours  in  which  to  make  the  journey,  this 


CONDITION   OF  THE  EARTH  15 

necessitates  360  meridians  on  the  earth  (24  x  60  =  1440 ; 
1440  ^  4  =  360).  For  the  same  reason,  if  we  travel  toward 
the  west  or  the  east,  we  find  the  time  to  be  constantly 
changing,  the  rate  of  change  being  4  minutes  for  each 
degree  of  longitude.^  While  the  sun  has  been  an  hour 
above  the  horizon  with  us,  15°  west  of  us  it  is  just  rising. 

If  a  body  on  the  earth  at  the  equator  travels  over  a  distance  of 
25,000  miles  in  a  day,  going  at  the  rate  of  about  17  miles  a  minute, 
the  question  may  be  asked.  Why  are  not  air,  water,  and  all  movable 
bodies,  left  behind  and  hurled  into  space?  The  answer  is  that  gravity 
draws  all  things  towards  the  earth  and  binds  them  to  it.  They  are 
a  part  of  the  earth,  moving  with  it,  just  as  a  person  becomes  a  part  of 
a  train  which  is  whirling  along  at  the  rate  of  a  mile  a  minute.  If 
the  earth  could  suddenly  stop,  all  movable  bodies  would  be  hurled 
away,  just  as,  when  a  train  suddenly  stops,  the  people  are  thrown 
forward.      / 

Revolution :  The  Sun  in  the  Heavens.  —  Though  rising 
and  setting  every  da}^,  the  sun  each  morning  rises  in  a  dif- 
ferent place  from  that  of  the  preceding  day.  This  is  scarcely 
noticeable  in  two  succeeding  mornings,  but  from  month 
to  month  is  distinctly  seen.  The  sun  slowly  changes 
its  path  through  the  heavens,  now  being  low,  again  high 
at  midday;  and  as  this  path  changes,  our  seasons  vary. 
In  about  365  days  (365.24  days)   the  cycle  of  .  seasonal 

1  In  this  country,  in  order  to  avoid  the  confusion  resulting  with  every 
town  having  its  own  true  or  solar  time,  artificial  boundaries  have  been 
drawn  parallel  to  the  meridians,  so  that  at  distances  of  15°  the  time 
changes  one  hour,  while  all  places  between  two  such  meridians  have  the 
same  standard  time.  There  are  several  such  belts,  and  now,  when  we 
travel  across  the  country,  we  are  obliged  to  set  our  watches  when  we 
come  to  the  boundaries,  setting  them  back  on  the  journey  west  and  ahead 
when  going  eastward.  In  this  country  there  are  five  divisions  of  Standard 
time  as  follows:  Intercolonial  (52i^-67i°),  Eastern  (67^°-82i°),  Central 
(82^°-97^°),  Mountain  (97^°-112n,  and  Pacific  (112L°-127^°). 


16  FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 

changes  —  the  year^  we  call  it  —  has  been  passed  through  ; 
and  then  we  again  go  over  the  same  cycle. 

If  we  could  spend  a  year  at  the  equator  and  others  at 
various  points  between  this  and  the  poles,  we  should  find 
an  entire  difference  in  the  seasons  of  the  several  places. 
In  each  place  the  sun  wouFd  have  a  new  series  of  move- 


FiG.  8. 
To  illustrate  day  and  night  at  equinox,  when  the  sun's  rays  reach  both  poles. 

ments,  but  in  a  single  locality^  the  cycle  would   be   the 
same  year  after  year. 

At  the  equator  the  sun  would  rise  nearly  in  the  east, 
pass  almost  directly  overhead  at  noon,  and  set  in  the  west. 
During  the  season  which  corresponds  with  our  winter, 
the  midday  sun  would  be  somewhat  south  of  the  zenith, 
and  during  the  season  corresponding  to  our  summer,  it 
would  be  an  equal  distance  north  of  the  vault.  Passing 
23J°  northward,  we  should  find  the  sun  to  be  always  south 


CONDITION  OF  THE  EABTB  17 

of  the  zenith,  excepting  in  midsummer,  when  it  would  ex- 
actly reach  the  zenith  at  midday ;  in  midwinter  it  would 
be  furthest  south.  Passing  north  of  this,  the  sun  would 
always  be  found  in  the  southern  half  of  the  sky.  At 
midday,  in  winter,  it  would  be  very  low,  while  at  the  same 
time,  the  days  would  be  short  and  the  nights  long. 

These  conditions  continue  to  increase  until  within  23|° 
of  the  pole,  where  the  midsummer  sun  rises  fairly  high 
in  the  heavens,  but  in  midwinter  just  reaches  the  southern 
horizon.  Beyond  this  the  sun  does  not  generally  have  a 
daily  rising  and  setting,  but  remains  above  the  horizon  for 
weeks,  and  further  north  for  months  at  a  time.  Then  it 
passes  below  the  horizon  to  stay  during  the  long,  cold 
winter  night.^  What  is  said  of  the  northern  hemisphere 
is  equally  true  for  the  southern,  if  we  change  south  to 
north,  and  north  to  south.  Our  winter  is  the  southern 
summer,  and  vice  versa. 

Cause  of  Seasons.  —  These  peculiarities  are  the  result 
of  a  second  movement  of  the  earth,  its  revolution  around  the 
sun.  Although  the  sun  is  on  the  average  about  92,800,000 
miles  distant,  the  tie  of  gravitation^  which  extends  through- 
out the  solar  universe,  keeps  the  sun  and  earth  together, 
while  the  latter  revolves  around  the  former,  whirling 
through  space  at  the  rate  of  1000  miles  a  minute,  and 
making  the  complete  journey  in  a  year,  and  year  after  year 
going  over  approximately  the  same  path.  If  it  were  not 
for  the  revolution,  and  the  earth  were  merely  a  rotating 

1  During  the  summer  the  sun  circles  near  the  horizon,  dipping  toward 
it  at  night,  when  it  is  near  the  north  (Fig.  71),  and  rising  higher  at  mid- 
day, when  it  has  circled  into  the  southern  quadrant.  Between  the  winter 
night  and  summer  day  there  are  short  seasons  when  the  sun  does  actually 
rise  and  set.  Exactly  at  the  pole  the  sun  is  above  the  horizon  half  the 
year,  and  below  it  the  other  half. 

0 


18  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

sphere  fixed  in  space,  we  could  have  no  seasons,  but  each 
day  would  be  like  the  preceding.  The  same  would  be 
true  if  the  earth  revolved  about  the  sun  with  its  axis  ver- 
tical to  the  plane  of  revolution.  Then  the  sun's  rays 
would  reach  the  equator  over  the  zenith  at  noon  of  every 
day  in  the  year,  and  north  of  the  equator,  at  any  given 


1 

■ 

■ 

■ 

^ 

^BP  A  .^, 

dUBiW 

^^^^^^H 

wr>     . 

"li**®^^^ 

^^^^^^^^^^^^H 

r- 

^fe^ 

% 

"-^ 

1 

4 

— -^-.^ 

• 

iiiita 

^r^^^^^H 

Fig.  9. 

To  illustrate  conditions  in  northern  summer  when  the  whole  Arctic  is 
bathed  in  sunlight. 

place,  every  day  would  find  the  sun  in  the  same  part  of 
the  heavens,  and  the  sunrise  and  sunset  would  always  be 
at  the  same  place.  As  the  poles  were  approached,  the  sun 
would  be  lower  and  lower  in  the  heavens,  until  at  the 
exact  pole,  it  would  be  seen  making  a  complete  circuit  of 
the  horizon.  This  is  exactly  what  happens  twice  each 
year,  at  the  time  of  the  vernal  and  autumnal  equinoxes 
(the  spring  and   autumn,  March   21   and   September   22 


CONDITION  OF  THE  EABTH 


19 


respectively),  when  the  day  and  night  are  equal  in  length, 
each  being  12  hours  (Fig.  8). 

In  reality,  the  earth  revolves  with  its  axis  inclined  at  an 
angle  of  about  23J°  (exactly  23°  27'  21'0  to  the  plane  of 
revolution,  and  it 
is  because  of  this 
that  we  have  the 
seasons.  Imagine 
the  earth  fixed  in 
space  and  rotating 
about  an  axis  in- 
clined 231^°  to  the 
plane  of  revolution 
of  the  earth  about 
the  sun.  Suppose 
that  the  North  Pole 
is  inclined  away 
from  this  plane, 
and  the  South  Pole 
toward  it  (Fig.  10). 
Then  the  sun  will 
be  vertical  at  Lat. 


Fig.  10. 

Condition  during  northern  winter  when  the  sun's 

rays  just  reach  the  Arctic  circle. 


23 J°  south  of  the  Equator,  or  over  the  Tropic  of  Capricorn. 
The  whole  of  the  south  polar  region  will  be  bathed  in 
light,  giving  perpetual  day  in  that  part  of  the  earth.  All 
the  southern  hemisphere  would  be  light.  In  the  northern 
hemisphere  the  sun  will  everywhere  be  in  the  southern 
heavens,  and  at  a  distance  of  23^°  from  the  North  Pole, 
the  solar  rays  will  cease  to  light  the  earth,  while  beyond 
this  line,  which  forms  the  Arctic  circle^  perpetual  night 
will  prevail. 

If  on  the  other  hand,  the  axis  is  turned  with  the  North 


20  FIRST  BOOK  OF  PHYSICAL   GFOGRAPHT 

Pole  toward  the  sun  (Fig.  9),  the  reverse  will  be  true,  and 
the  sun's  rays  will  always  be  vertical  at  noon  over  the 
northern  tropic,  Cancer,  which  is  23 1°  north  of  the  Equator. 
Beyond  the  Antarctic  circle,  or  a  distance  of  23|-°  from  the 
South  Pole,  a  condition  of  perpetual  night  is  present.  If 
this  position  were  maintained,  the  one  hemisphere  would 
have  perpetual  winter,  the  other  perpetual  summer ;  and 
the  temperature  would  decrease  from  that  tropic  over 
which  the  sun  was  vertical,  toward  each  pole. 

Since  the  axis  is  inclined,  and  is  year  by  year  pointing 
toward  nearly  the  same  place  in  the  heavens^  (the  north 
pole  pointing  approximately  toAvard  the  North  Star),  the 
revolution  of  the  earth  about  the  sun  turns  the  North 
Pole  now  toward  and  now  away  from  the  sun  (Fig.  11), 
and  so  the  two  hemispheres  enjoy  alternation  of  seasons, 
and  are  thus  treated  alike.^  This  is  the  same  as  saying, 
that  when  the  South  Pole  is  turned  from  the  sun,  and 
when  winter  prevails  in  the  southern  hemisphere,  we  in 
the  northern  hemisphere  have  long  summer  days  with  the 
sun  high  in  the  heavens  at  noonday.  This  then  gradually 
changes  to  autumn,  when  the  days  become  equal  in  length, 
and  our  sun  is  less  high  in  the  heavens.  At  this  time,  the 
rays  of  the  sun  cover  the  entire  earth,  which  is  the  condi- 


^In  the  course  of  long  periods  of  time  this  does  change,  but  this  is  a 
question  of  astronomy  which  does  not  bear  especially  upon  the  present 
subject. 

2  It  is  very  commonly  the  case  that  the  pupil  merely  memorizes  these 
facts  without  really  grasping  the  fundamental  principles  ;  but  the  teacher 
should  see  to  it  that  each  student  really  understands  these  points.  This 
can  be  easily  done  if  the  teacher  will  make  intelligent  use  of  a  globe, 
or  of  any  spherical  body,  showing  the  way  in  which  the  axis  maintains 
its  position  while  the  earth  moves,  and  the  hemispheres  face  toward  and 
away  from  the  sun  in  the  different  seasons. 


CONDITION  OF  THE  EARTH 


21 


tion  that  would   exist   if    the  earth's  axis  were  at  right 
angles  to  the  plane  of  revolution. 

Gradually  the  sun  takes  a  lower  position  in  the  heavens, 
the  day  shortens,  and  midwinter  is  reached,  while  at  the 
same  time  summer  prevails  south  of  the  Equator.  Then 
begins  the  return  of  warmth  with  the  spring  and  lengthen- 


Wmter 
Solstice 


VemaLEqumox 

Fig.  11. 

To  show  change  in  seasons  as  the  earth  revolves  about  the  sun  (ellipse  exag- 
gerated and  relative  size  and  distance  not  shown). 

ing  days.  Soon  the  yearly  cycle  is  over,  because  the  revo- 
lution is  complete,  and  the  sun  has  come  back  nearly  to 
the  place  where  it  started  the  year  before.  As  soon  as 
one  revolution  is  finished  a  new  one  is  begun,  and  so  year 
after  year  the  earth  pursues  its  path  about  the  sun,  and 
year  by  year  we  find  the  same  alternation  of  seasons.  So 
distinct  is  this  cycle,  that  astronomers  are  able  to  predict 
just  what  the  position  of  the  sun,  and  the  length  of  the  day, 
will  be  a  hundred  years  from  now. 


CHAPTER   II 

THE   UNIVERSE 

The  Solar  System:  The  Sun.  —  The  earth  is  but  one  of 
a  great  family,  all  having  certain  resemblances,  and  all 
bound  together  by  the  common  bond  of  gravitation.  They 
pass  through  space  ^  in  company,  yet  each  performs  certain 
duties  and  movements  of  its  own.     The  central  body  is  the 


Fig.  12. 

To  show  that  of  all  the  light  and  heat  sent  from  the  sun  in  all  dii-ectious,  the 
earth  receives  but  a  very  little. 

^  Space  is  the  great  unknown  expanse  which  surrounds  the  earth,  and 
so  far  as  we  know,  extends  without  limit  in  all  directions.  We  are  unable 
to  conceive  of  anything  without  an  end,  and  yet  we  are  unable  to  con- 
ceive of  an  end  to  space :  space  bafiles  our  most  acute  perception.  It  is 
believed  to  be  empty  of  all  substances  with  which  we  are  acquainted ;  yet 
since  light  and  heat  pass  through  it,  it  is  thought  to  be  pervaded  by  a 
mysterious  ether,  which  allows  waves  of  light  and  heat  to  pass  from  the 
sun  to  the  earth.  Its  temperature  is  believed  to  be  very  low,  perhaps  200 
or  300  degrees  below  zero. 

22 


THE   UNIVERSE  23 

sun,  a  hot  glowing  mass,  apparently  composed  of  the  same 
elements  as  the  earth  itself,  but  so  highly  heated  that  both 
heat  and  light  are  emitted  in  all  directions  into  space 
(Fig.  12).  A  small  part  of  this  is  intercepted  by  the 
earth  as  it  moves  around  the  sun,  and  this  form  of  energy 
performs  work  of  immense  importance.  Like  the  earth 
itself,  the  sun  is  a  great  spherical  body,  its  diameter  being 
about  860,000  miles,  or  more  than  100  times  that  of  the 
earth.  If  the  centre  of  the  sun  were  within  the  earth, 
its  body  would  not  only '  cover  all  the  space  between 
us  and  the  moon,  but  it  would  extend  two-thirds  as  far 
beyond. 

The  Planets}  —  While  the  earth  revolves  around  the 
central  sun,  and  receives  its  light  and  heat  from  this 
source,  it  is  not  alone  in  this,  for  there  are  other  great 
spheres  which  also  revolve  about  the  sun  in  orbits  which 
bear  a  general  resemblance  to  that  pursued  by  the  earth. 
These  are  called  planets^  and  there  are  eight  of  these,  which, 
named  in  the  order  of  their  distance  from  the  sun,  are  Mer- 
cury, Venus,  Earth,  Mars,  Jupiter,  Saturn,  Uranus,  and 
Neptune.  They  are  all  somewhat  flattened  spheres  (oblate 
spheroids),  revolving  in  nearly  circular  elliptical  paths 
about  the  sun,  and  those  that  are  well  enough  known 
have  been  found  to  have  a  rotation  about  an  axis.  In  the 
heavens  they  shine  as  stars ;  but  their  light  is  reflected 
from  the  sun.  Some,  at  least,  have  an  atmosphere,  but  at 
present  our  knowledge  of  most  of  the  planets  is  very  slight. 

Jupiter,  the  largest  of  these  (86,000  miles  in  diameter), 

1  Besides  the  planets  there  are  smaller  spheres,  called  asteroids,  in  the 
space  between  Mars  and  Jupiter.  The  largest  is  about  520  miles  in  diam- 
eter, and  the  smallest  less  than  40  miles.  There  are  also  comets  and  shoot- 
ing stars  moving  in  the  space  occupied  by  the  solar  system. 


24  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

has  a  mass  greater  than  all  the  others  combined ;  but  it  has 
only  one-tenth  the  diameter  of  the  sun.  On  the  other  ex- 
treme, Mercmy,  the  planet  nearest  the  sun,  has  a  diameter 
of  about  2992  miles,  being  only  a  little  less  than  one- 
half  that  of  the  earth.  The  other  planets  range  in  size 
between  these  two  extremes.  While  the  distance  of 
the  earth  from  the  sun  averages  about  92,800,000  miles, 


Fig.  13. 

Diagram  to  show  relative  distance  of  the  planets  from  the  sun,  and  also 
their  relative  sizes. 


Mercury  is  only  35,750,000  miles  distant,  Jupiter  about 
480,000,000,  and  Neptune,  the  most  distant  of  the  planets, 
is  2,775,000,000  miles  away.  In  travelling  through  these 
immense  distances,  in  their  journey  about  the  sun,  the 
earth  occupies  365  days.  Mercury  88  days,  Jupiter  12  of 
our   years,  and  Neptune  about  165  years. 

Satellites.  —  While  each  of  the  planets  is  revolving 
about  the  sun,  all  but  two  of  them.  Mercury  and  Venus, 
have  smaller  bodies  revolving  around  them.  These  Satel- 
lites vary  in  number,  Saturn  having  eight. 

The  earth's  satellite,  the  moon,  is  a  cold  sphere  with  a 
diameter  of  about  2160  miles,  and  an  average  distance  from 
the  earth  of  about  240,000  miles.  In  company  with  the 
earth  it  moves  about  the  sun,  and  as  it  goes,  makes  the 
journey  around  the  earth  every  29|-  days.  When  it  shines, 
it  does  so  by  reflected  sunlight. 


THE   UNIVERSE 


25 


Being  so  near  the  earth,  the  moon  has  been  carefully 
studied  by  the  aid  of  powerful  telescopes,  and  we  know 
more  about  its  surface  than  we  do  about  any  other  body  in 
space.  Only  one  side  is  turned  towards  us,  and  we  never 
see  the  opposite  face.  On  that  side  which  we  see  there  is 
neither  water  nor  atmosphere  :  its  surface  is  very  rough, 


IT 

m 

p3i,  "^^,.';^^3 

^ 

IK^^B 

^ 

MESM 

^3S#*^'^           ■ 

Jv  ■-■■■ 

0^% 

r"- 

T-     -•-■.' 

■    .  ■       -:■-•:  •^^-"-•.>A,>.,„..| 

Fig.  14. 

CruLers  ;md  rid<res  on  the  moon. 


and  many  of  the  irregularities  are  great  crater-like  pits,  re- 
sembling immense  volcanic  craters  (Fig.  14).  It  is  thought 
by  many  that  these  are  craters  of  ancient  volcanoes  which 
are  now  extinct.  Thousands  of  them,  great  and  small, 
pit  the  surface  of  the  moon. 

The  Universe.  —  While  the  earth  seems  so  large  to  us, 
and  while  most  of  us  see  but  a  tiny  part  of  it,  and  know  of 
the  rest  only  from  the  description  of  others,  it  is,  relatively, 
but  a  tiny  speck  of  dust  in  the  great  universe  upon  which 
we  gaze  every  starry  night.  When  we  look  upon  a  twink- 
ling star,  we  see  a  sun  so  distant  that  the  very  light  which 


26  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

meets  our  eye  may  have  left  the  star  hundreds  of  years 
ago ;  and  perchance  the  star  that  we  see,  no  longer  exists. 

In  the  Milky  Way  also,  which  to  the  unaided  eye  looks 
like  the  gleam  of  the  sun's  rays  reflected  from  a  thin  cloud 
in  the  upper  air,  the  telescope  finds  myriads  of  stars,  one 
beyond  the  other ;  and  beyond  these  are  still  others  whicli 
even  the  telescope  cannot  distinguish.  In  this  belt  of 
abundant  star-dust,  the  limit  of  suns  cannot  be  told ;  and 
yet  all  of  these  bodies  are  so  fixed  in  space  that  year  by 
year  their  position  appears  to  be  unchanged. 

What  is  this  space,  the  nature  of  which  no  man  can 
even  guess,  and  the  limits  of  which  no  man  has  yet  been 
able  to  find?  Can  those  who  believe  that  they  know  the 
origin  of  the  universe,  and  who  think  that  they  can  re- 
duce it  to  a  system  of  natural  laws,  throw  any  light  upon 
this?  If  so,  they  have  yet  to  announce  an  explanation 
which  satisfies  the  average  mind.  There  is  something  in 
this  wonderful  system  that  may  well  cause  tlie  human 
mind  to  recognize  its  own  smallness  and  insignificance. 

In  the  heavens  there  are  large  clusters  of  stars,  which 
to  the  QjQ  are  unknown,  but  which  the  telescope  reveals ; 
and  these  may  be  other  stellar  systems^  like  that  which  we 
view  when  we  look  into  the  star-lit  vault  of  the  heavens. 
Is  our  stellar  system,  of  which  the  great  solar  system  is 
but  a  small  part,  itself  a  small  portion  of  a  great  universe 
of  many  stellar  systems  ?  Here  again  no  answer  can  be 
given.  Are  the  tiny  stars  each  a  mother  sun,  with  a  fam- 
ily of  planets  ?  and  if  so,  do  these  planets  resemble  ours, 
and  are  they  inhabited  by  life  ?  Again  we  cannot  even, 
guess :  but  why  may  not  this  be  so ;  for  is  it  probable  that 
in  all  this  great  and  apparently  endless  universe,  our  tiny 
earth  is  the  only  favored  spot? 


TUB   UNIVERSE  27 

The  power  of  the  human  mind  is  indeed  restricted,  for 
we  learn  by  experience.  The  infant  reaches  out  to  grasp 
objects  that  are  far  beyond  its  reach ;  the  child  of  two  or 
three  will  try  to  make  an  object  pass  through  a  space 
smaller  than  itself,  and  will  learn  better  only  by  repeated 
experiments ;  the  boy  of  ten  or  fifteen,  who  has  known 
only  his  own  town  or  country,  can  have  only  a  slight  con- 
ception of  the  size  of  the  earth ;  and  the  man,  accustomed 
to  measure  by  his  experiences  on  the  earth,  can  have  no 
proper  conception  of  the  distance  of  the  sun,  and  cannot 
even  dream  of  the  meaning  of  a  billion  miles. 

The  best  that  one  can  do,  in  lieu  of  our  inability  to  really  conceive 
this,  is  to  become  impressed  with  the  immensity  of  these  distances  of 
space  by  some  comparison  with  things  of  ordinary  experience.  An 
express  train  in  most  cases  goes  no  faster  than  60  miles  an  hour, 
and  as  it  passes  us,  it  comes  and  goes  with  a  rush  that  is  almost 
startling.  Let  us  suppose  that  we  could  start  on  a  journey  from  the 
sun  to  Xeptune,  passing  the  earth,  and  the  moon,  and  travelling  con- 
tinuously at  the  rate  of  60  miles  an  hour.  At  this  rate  of  speed,  it 
would  take  17  days  to  travel  around  the  earth  at  the  Equator.  Start- 
ing at  the  sun,  a  little  more  than  176  years  would  be  occupied  in  reach- 
ing the  earth.  A  little  more  than  166  days  would  take  us  to  the  moon, 
and  the  journey  from  the  sun  to  the  planet  Neptune  would  require 
5280  years.  To  reach  the  nearest  star  would  take  several  hundred  times 
as  many  years.  That  is  to  say,  if  one  had  started  from  the  sun  at  the 
beginning  of  the  Christian  era,  he  would  still  be  journeying,  and 
would  be  only  part  way  between  Saturn  and  Uranus  ;  yet  during  this 
period  of  time  a  great  part  of  the  recorded  history  of  the  human  race 
has  taken  place. 

The  Nebular  Hypothesis  :  Symmetry  of  the  Solar  System. 
—  Reviewing  the  conditions  shown  by  astronomers  to 
exist  in  the  solar  system,  it  is  found  that  the  regular 
members  are  all  spherical  bodies,  and  those  that  arc  near 


28  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

enough  to  have  been  studied  carefully,  show  a  flattemng 
in  the  polar  regions.  All  that  are  well  enough  known, 
show  a  rotation  about  an  axis  passing  through  these 
flattened  parts  of  the  sphere;  and  all  of  them  revolve 
about  the  sun  in  the  same  direction.  The  axes  of 
rotation  are  all  inclined  to  the  plane  of  revolution.  The 
satellites  show  similar  uniformity ;  and  in  addition  they 
are  revolving  around  their  parent  planets.  These  move- 
ments are  all  so  regular  that  astronomers  can  predict  in 
advance  exactly  what  they  will  be.  The  paths  pursued 
by  these  bodies  are  all  nearly  circular  ellipses,  at  one  of 
the  foci  of  which  is  situated  the  central  body,  the  sun, 
around  which  the  revolution  is  made.  There  is  therefore 
a  wonderful  symmetry  of  form  and  movement  of  the 
spheres ;  all  obey  the  laws  of  gravitation,  by  which  they 
are  all  bound  together  in  this  regular,  well-established 
system  of  movement. 

Tliere  is  harmony  also  in  other  respects.  Astronomy 
tells  us  something  of  the  composition  of  the  sun,  and  in 
this  are  found  some  of  the  very  elements  which  compose 
the  earth.  There  appears  to  be  a  progressive  decrease  in 
heat  as  the  size  of  the  sphere  diminishes.  The  sun,  the 
largest,  is  intensely  hot ;  Jupiter,  next  in  size,  is  apparently 
warm,  but  is  not  luminous  at  the  surface ;  the  earth  is 
cold  at  the  surface,  and  hot  within ;  the  moon  appears  to 
be  cold  throughout  its  entire  mass.  Again,  from  Mercury, 
the  planet  nearest  the  sun,  to  Neptune,  the  most  remote, 
there  is  an  almost  uniform  decrease  in  density  of  the  ma- 
terials composing  the  planets. 

The  Exj^lanation.  —  It  has  seemed  to  men  that  these 
conditions  called  for  a  uniformity  of  origin ;  and  before  all 
of  these  fncts  were  known,  philosophers  and  astronomers 


TUB   UNIVERSE  29 

had  proposed  the  brilliant  explanation  for  the  solar  system 
which  \Ye  know  as  the  Nebular  Hypothesis.  This  is  still 
held  by  astronomers,  and  many  new  facts  have  been 
brouglit  to  its  support.  While  it  cannot  be  said  to  be 
more  than  a  theory,  it  has  the  advantage  of  explaining 
nearly  all  the  facts,  while  there  is  little  to  oppose  it.  It 
is  now  more  firmly  grounded  than  ever  before. 

Briefly,  the  Nebular  Hypothesis  is  this:  In  the  begin- 
ning, the  solar  system  was  a  mass  of  glowing  gas,  slowly 
revolving  in  the  direction  which  the  planets  now  pursue 


c 


/?^- 


••vii^  b 


"'•^•^:v-::^-^^""    Fig.  15. 

Diagram  to  illustrate  Nebular  Hypothesis.  A  mass  of  heated  gas  {A)  more 
deuse  at  ceuire  (c)  is  roiatiug  iu  the  direction  of  the  arrows ;  in  i^  a  riug 
ah  is  thrown  off,  more  dense  at  a  than  elsewhere;  in  C  this  ring  has  gath- 
ered {K)  around  a  centre  {a  iu  B)  and  has  itself  thrown  off  a  ring  (i), 
while  another  ring  {e)  has  come  off  from  the  central  mass;  iu  Z)  a  planet 
(P)  and  Satellite  (S)  have  formed,  and  these  are  revolving  in  the  direction 
of  the  arrows.  The  ring  (e  in  C)  have  also  gathered  into  spheres 
{MdindiN). 

in  their  movement  around  the  sun.  The  mass  was  cool- 
ing by  the  radiation  of  the  heat  into  space,  just  as  the  sun 
and  earth  are  now  still  losing  heat.  There  was  a  certain 
loss  of  bulk  from  contraction,  and  in  the  course  of  time 
this  caused  the  parent  mass  to  throw  off  rings,  some- 
thing like  those  which  rise  from  an  engine  as  it  is  start- 
ing from  a  railway  station.  These  continued  to  slowly 
revolve  in  the  original  direction,  and  gravity  gradually 
drew  the  mass  of  each  ring  together  into  a  spherical  bod}-, 
about   some    portion   which   was    originally   more    dense 


30  FIBST  BOOK  OF  PHYSICAL   GEOGBAPHY 

than  the  rest.  The  sphere  of  gas  continued  to  revolve 
about  the  parent  nebula,  and  to  rotate  as  it  went.  Some 
of  these  spheres  have  themselves  thrown  off  rings,  which 
upon  passing  through  the  same  history  began  to  form 
spheres,  which  revolved  around  their  parents. 

As  the  heat  became  less  intense,  in  the  course  of  time 
these  began  to  solidify,  the  smallest,  and  those  that  were 
first  thrown  off,  being  the  first  to  reach  the  solid  stage. 
Therefore  the  sun,  which  is  the  largest  and  most  central 
bod}^,  is  the  hottest,  while  so  small  a  body  as  the  moon, 
and  so  distant  a  planet  as  Neptune,  are  the  coldest. 

Facts  accounted  for. —  This  hypothesis  accounts  for  the  uniformity 
of  rotation  and  of  revolution  :  it  explains  the  spherical  form,  because 
gravity,  acting  upon  a  gaseous  body  will  necessarily  produce  a  sphere. ^ 
It  explains  the  flattening  at  the  poles,  because,  by  the  centrifugal 
force,  a  rotating  sphere  of  gas,  or  of  liquid,  will  bulge  at  the  Equator, 
where  the  rotation  is  most  rapid.  It  accounts  also  for  the  heat  of  the 
sun  and  the  earth's  interior.  It  also  explains  the  decrease  in  density 
from  the  inner  to  the  outer  members  of  the  system  ;  for  the  first  rings 
thrown  off  would  be  composed  of  the  less  dense  outer  portions  of  the 
nebula  (just  as  the  air  and  water  of  the  earth  are  outside  of  the 
denser  crust);  and  finally,  it  tells  why  the  sun  and  the  earth  contain 
the  same  elements.  At  the  same  time  it  must  be  understood  that 
this  satisfactory  explanation  depends  upon  some  very  distinct  assump- 
tions, and  it  presupposes  that  there  loas  a  nebulous  mass  of  hot  gas, 
revolving  and  under  the  influence  of  gravitation. 

If  this  explanation  is  correct,  the  earth  is  descended  from  a  much 
hotter  body,  and  has  now  reached  a  stage  in  cooling  when  only  the 
interior  is  hot;  and  it  will  continue  to  lose  heat  until  the  condition  of 
the  moon  is  reached.  In  time,  in  the  course  of  indefinite  ages,  the 
sun  also  will  lose  its  heat,  and  our  globe  will  be  cut  off  from  the  supply 
of  heat  and  light  energy  which  are  of  such  vital  importance  to  all  life. 

1  This  may  be  illustrated  by  a  globule  of  oil  in  water,  and  the  flatten- 
ing may  be  shown  by  revolving  such  a  sphere. 


THE   UNIVERSE  31 

Other  Nehuloe.  —  Far  away  in  space,  the  telescope  has 
revealed  masses  of  glowing  gas  like  that  which  the  Neb- 
ular Hypothesis  conceives ;  and  some  of  them  show  the 
condensing  rings  and  spheres,  like  those  supposed  to  have 
existed  when  the  solar  nebula  was  forming.  From  this  it 
seems  possible,  that  in  the  far-away  confines  of  space,  other 


Fig.  16. 

The  Andromeda  nebula  showing  rings  and  denser  parts  in  a 

nebulous  mass. 

worlds  are  even  now  in  process  of  formation.  Whether 
true  or  not,  it  is  a  beautiful  hypothesis.  It  is  an  attempt 
of  the  human  mind  to  explain  the  most  profound  mystery 
of  nature,  —  to  account  for  the  wonderful  law  and  sym- 
metry that  everywhere  prevails;  but  the  mind,  although 
always  impelled  to  attempt  explanation,  is  liable  to  error, 
and  is  very  limited  in  its  power  of  conception. 


PART   II.  — THE   ATMOSPHERE 

CHAPTER   III 

GENERAL  FEATURES  OF  THE  AIR 

Importance  of  the  Air.  —  An  invisible  ocean  of  elastic 
gas  surrounds  the  earth  with  its  life-giving  substance. 
It  fans  the  surface  with  breezes  and  disturbs  it  with 
violent  winds.  It  carries  invisible  vapor  from  water  to 
land,  where  it  falls  as  rain.  It  spreads  light  and  warmth 
over  the  globe,  and  in  many  ways  its  presence  works  to 
the  advantage  of  the  varied  life  that  overspreads  the  earth. 
Although  invisible,  the  air  has  substance,  and  when  it  is 
in  motion  we  feel  the  breeze  or  wind.  We  breathe  it, 
and  it  gives  to  us  materials  that  are  necessary  for  our 
existence. 

Place  an  animal  in  a  small  enclosed  space,  and  it  soon  exhausts  the 
air,  and  although  a  gas  still  remains,  it  is  different  from  the  original. 
The  breathing  has  caused  a  chemical  change,  and  unless  new  air  is 
furnished  the  animal  dies.  A  candle  placed  in  a  similar  position  will 
soon  cease  to  burn,  and  it  is  then  found  that  the  gases  of  the  air  have 
been  changed  by  the  burning  of  the  candle. 

If  an  animal  should  be  placed  under  a  closed  cylinder  on  an  air 
pump,  and  the  air  be  exhausted,  death  would  soon  result,  for  there 
would  be  no  air  to  breathe  and  perform  the  work  which  the  body  con- 
stantly demands  of  it.  For  a  similar  reason  a  person  suffocates  and 
drowns  when  kept  for  a  few  minutes  under  water,  which  excludes  the 
air  from  the  lungs. 

32 


GENERAL  FEATURES   OF  THE  AIR  33 

Composition  :  Oxygen  and  Nitrogen.  —  Careful  study  has 
shown  that  the  air  is  made  chiefly  of  two  gaseous  elements, 
nitrogen  and  oxygen^  about  21%  of  the  latter  to  79%  of  the 
former.^ 

In  the  air,  nitrogen  (and  also  argon)  is  a  very  inert  ele- 
ment, which  acts  as  an  adulterant  to  tlie  active  oxygen, 
in  a  manner  similar  to  the  adulteration  or  weakening  of  a 
solution  of  salt  when  water  is  added  to  it.  It  is  oxygen 
that  is  doing  the  work  in  the  bodies  of  animals,  and  caus- 
ing many  changes  on  the  earth.  Nevertheless  nitrogen 
is  a  very  important  part  of  the  atmosphere ;  for  if  it  were 
absent,  the  bulk  of  the  air  would  be  very  much  less,  and 
tlie  work  of  the  oxygen  more  rapid.  Experiments  have 
shown  that  an  animal  cannot  live  in  pure  oxygen,  under 
a  pressure  greater  than  that  of  the  air.^ 

Carbonic  Acid  Gas.  —  In  a  burning  candle  or  lamp,  the 
oxygen  of  the  air  is  producing  a  chemical  change  in  the 
burning  substance.  If  we  should  exhaust  the  oxygen, 
the  light  would  go  out.  If  more  oxygen  were  added,  the 
light  would  burn  much  more  brilliantly.  This  combustion 
or  oxidation  is  somewhat  like  that  which  takes  place  when 
a  man  breathes  the  life-giving  oxygen  into  his  lungs,  for 
then  also  the  oxygen  gas  combines  with  other  substances. 

1  In  1894  a  new  gaseous  substance,  called  argon,  was  discovered  in 
the  atmosphere,  of  which  it  forms  a  considerable  proportion.  It  may- 
appear  strange  that  an  element  which  we  have  always  been  breathing 
should  so  long  have  escaped  detection  ;  but  this  new  element  resembles 
nitrogen  so  closely  that  the  two  have  been  confused.  At  present  it  is 
impossible  to  tell  much  about  the  new  gas. 

2  It  is  something  like  the  difference  between  a  fire  with  the  draft  closed 
and  one  with  an  open  draft,  through  which  more  oxygen  is  furnished,  thus 
causing  more  rapid  burning.  The  teacher  can  easily  show  this  by  an 
experiment,  making  and  collecting  oxygen  in  a  receiver  in  which  a  candle 
is  burning. 


34  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

In  a  lamp,  and  in  the  lungs,  oxygen  combines  with 
carbon,  producing  the  gas  which  is  known  as  carbonic  acid 
gas  (carbon  dioxide).  This  is  why  a  lighted  candle  in  a 
small  jar  soon  ceases  to  burn ;  for  after  awhile  all  of  the 
oxygen  is  consumed  by  combining  with  the  carbon  of  the 
candle,  and  its  place  is  taken  by  the  newly  made  carbonic 
acid  gas.  For  the  same  reason  an  animal  cannot  live  long 
in  a  closed  space  a  little  larger  than  itself. 

Growing  plants  perform  the  reverse  work  of  converting 
carbonic  acid  gas  back  to  oxygen.  They  need  carbon, 
and  they  take  it,  furnishing  in  return  pure  oxygen. 
Hence  in  a  measure  they  act  as  purifiers  of  the  atmos- 
phere, destroying  some  of  the  carbonic  acid  gas  made  by 
animals,  and  replacing  it  by  oxygen. 

But  carbonic  acid  gas  forms  an  appreciable  part  of  the  air 
(about  .03%  of  the  whole),  and  it  is  everywhere  present. 
There  are  several  sources  from  which  it  is  known  to  come. 
When  breathing,  every  animal  is  furnishing  some,  and 
everything  that  burns  supplies  this  gas  to  the  air.  Every 
animal  and  plant  that  is  dead  and  decaying  is  giving  out 
this  gas,  and  a  supply  is  also  obtained  from  the  earth 
itself.  Much  carbon  is  locked  up  in  the  earth,  as  for 
instance,  where  plants  have  not  decayed  but  have  been 
changed  to  mineral  coal.  We  burn  this  in  our  stoves, 
and  one  of  the  products  is  carbonic  acid  gas.  It  is  also 
constantly  escaping  from  many  springs,  and  probably 
also  from  the  soil.  Also  when  a  volcano  breaks  forth 
in  eruption,  large  quantities  of  this  gas  escape.  Because 
of  the  large  amount  of  fuel  burned  there,  more  carbonic 
acid  gas  exists  in  the  air  near  cities  than  in  the  open 
country. 

This  gas  serves  well  to  illustrate  how  beautifully  every- 


GENERAL  FEATURES  OF  THE  AIR  35 

thing  is  adjusted  to  the  existence  and  development  of  life 
on  the  earth.  Here,  for  instance,  is  a  gas,  forming  only 
.03%  of  the  entire  atmosphere,  which  if  decidedly  in- 
creased, or  slightly  diminished,  would  be  fatal  to  all 
animal  life  on  the  land.  If  very  much  increased,  it 
would  be  directly  fatal ;  if  diminished,  the  plant  life  that 
depends  upon  it  for  existence  would  perish;  and  as  the 
animals  of  the  land  cannot  take  their  food  directly  from 
the  earth,  but  obtain  it  entirely  by  means  of  plants,  the 
destruction  of  these  would  necessitate  the  death  of  all 
animals  excepting  those  that  could  draAV  entirely  upon 
the  ocean  for  their  subsistence.  But  for  untold  ages  this 
harmony  of  nature's  balance  has  been  maintained,  and  the 
earth  has  been  clothed  with  vegetation  and  occupied  by 
myriads  of  animals,  great  and  small,  c^ 

Water  Vapor.  —  While  there  are  minute  quantities  of 
many  other  gases  in  the  air,  there  is  but  one  other  really 
important  gaseous  constituent.  When  wet  clothes  are 
placed  upon  the  line  to  dry,  little  by  little  the  water  dis- 
appears, until  finally  none  is  left.  Also  after  a  rain,  the 
pools  of  water  in  the  road  slowly  disappear,  the  mud  dries 
up,  and  the  water  is  gone.  In  both  cases  it  has  evaporated, 
and  has  changed  its  form  from  the  visible  liquid  to  the 
invisible  gas  which  we  call  water  vapor  (Chapter  IX). 

This  process  of  evaporation  is  somewhat  like  that  which 
is  producing  steam.  The  kettle  on  the  stove  boils,  steam 
issues  from  the  neck,  and  in  time  the  kettle  becomes  dry, 
the  water  having  changed  to  vapor,  which  is  still  present 
in  the  air  of  the  room,  though  no  longer  to  be  seen.  That 
it  is  present  may  be  shown  on  a  frosty  day;  for  then  when 
the  vapor-laden  air  of  the  room  encounters  the  cold  win- 
dow, some  of  the  vapor  is  condensed  back  to  water,  forming 


36  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

drops  on  the  glass. ^  If  the  day  is  very  cold,  it  solidifies 
into  fantastic  frost  crystals,  the  solid,  icy  form  which  water 
takes  when  the  temperature  has  descended  below  the 
freezing  point.  Even  the  breath  may  furnish  enough 
vapor  to  cause  this,  and  on  cold  nights  our  chamber 
windows  are  covered  with  frost. 

The  housewife  knows  that  some  days  are  better  drying 
days  than  others.  When  the  warm  sun  shines  upon  the 
clothes,  they  generally  dry  quickly,  for  evaporation  takes 
place  more  rapidly  in  warm  than  in  cold  air.  But  heat  is 
only  one  of  the  aids  to  evaporation,  and  this  is  illustrated 
by  the  fact  that  some  of  the  hot,  muggy  days  of  summer 
are  not  such  good  drying  days  as  the  cold,  windy  spells 
of  winter.  This  is  because  the  air  cannot  contain  more 
than  a  certain  quantity  of  vapor,  and  on  the  muggy 
summer  days  the  air  is  nearly  saturated,  while  it  is  rela- 
tively dry  daring  the  cold,  windy  days  of  winter.  Because 
it  moves  the  air,  the  wind  also  favors  evaporation,  and 
thus  does  not  allow  it  to  remain  near  the  damp  object 
long  enough  to  become  saturated. 

The  amount  of  water  vapor  that  the  air  can  contain 
depends  upon  the  temperature,^  warm  air  being  able  to 

1  The  teacher  may  illustrate  this  by  bringing  a  pitcher  of  ice  water  into 
a  warm  room. 

2  The  comparison  may  be  made  (though  it  is  only  partly  analogous)  to 
a  sugar  solution.  If  several  spoonfuls  of  sugar  are  placed  in  a  dish  of 
oold  water,  all  of  it  may  not  dissolve.  Heating  this  water,  more  sugar 
is  taken  into  solution,  and  then  if  the  sugar  water  is  allowed  to  cool,  some 
of  the  dissolved  sugar  will  be  precipitated  in  the  form  of  crystals.  So  it 
is  with  the  air  ;  cold  air  can  contain  little  vapor,  warm  air  will  hold  more ; 
and  if  this  is  then  cooled,  some  of  the  vapor  may  be  forced  to  assume  the 
liquid  or  solid  forms  of  rain,  fog,  dew,  or  frost. 

When  saturated,  at  ordinary  pressure,  a  room  10  feet  high  and  20  feet 
square  contains  34(5  pounds  of  air,  if  the  temperature  is  0^.     In  this,  if 


GENERAL  FEATUBES   OF  TUE  AIR  37 

carry  more  than  cold.  Nevertlieless,  air  will  carry  some 
vapor  even  when  its  temperature  is  below  the  freezing 
point.  This  is  illustrated  on  a  cold  winter  day,  when  the 
clothes  freeze  upon  the  line  but  still  continue  to  dry. 
The  rate  of  evaporation  depends  partly  upon  the  dryness 
of  the  air;  for  just  as  a  saturated  solution  of  salt  cannot 
be  made  stronger  without  increasing  the  temperature,  so 
air,  having  as  much  vapor  as  it  can  hold,  will  take  no  more, 
while  dry  air  greedily  absorbs  it.^  Hence  dry  air  evapo- 
rates water  more  rapidly  than  nearly  saturated  or  humid 
air.  If  of  high  temperature,  more  can  be  evaporated  than 
at  lower  temperatures,  and  if  moving,  it  takes  vapor  more 
readily  than  if  quiet. 

Although  present  in  very  small  quantities,  forming 
only  a  small  projDortion  of  the  entire  atmosphere,  water 
vapor  is  one  of  the  most  important  constituents  of  the  air. 
Even  in  the  driest  parts  of  the  land  it  is  always  present, 
although  then  in  very  small  quantities.  The  conversion 
of  water  vapor  back  to  water  or  snow  is  constantly  in 
progress.  Every  cloud,  every  fog  particle,  every  glisten- 
ing drop  of  dew,  and  every  drop  of  rain  or  snow  crystal,  is 
a  witness  of  this  remarkable  transformation;  and  as  it 


saturated,  there  is  an  amount  of  vapor  which,  transformed  to  water,  would 
weigh  one-third  of  a  pound.  If  the  temperature  is  increased  to  60°,  and 
the  air  still  saturated,  its  weight  is  301  pounds,  and  the  vapor  when  con- 
densed would  weigh  '6\  pounds.  If  the  temperature  is  raised  to  80°,  the 
air  weighs  291  pounds,  and  the  vapor  if  condensed,  Q\  pounds.  A  pound 
of  water  equals  about  one  pint. 

1  For  purposes  of  graphic  description  it  is  convenient  to  make  this  com- 
parison ;  yet  physicists  know  that  evaporation  would  occur  if  there  were 
no  air,  for  it  depends  not  upon  air,  but  upon  the  water ;  but  the  air  is 
important  in  evaporation,  because  it  bears  the  vapor  away  and  also  warms 
the  water  by  its  presence. 


38  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

silently  and  almost  mysteriously  proceeds  in  the  change, 
a  work  of  vital  importance  is  performed. 

It  sprinkles  the  land  with  showers,  causing  countless 
myriads  of  plants  to  burst  forth  into  leaf,  flower,  and  fruit. 
It  transforms  the  salt  water  of  the  sea  to  fresh  drops  of  rain ; 
and  this,  in  our  rivers,  lakes,  and  springs,  furnishes  the 
water  supply  upon  which  we  are  so  dependent.  Without 
this  ingredient  of  the  great  atmospheric  ocean,  the  earth 
would  be  a  desert  sphere  whirling  aimlessly  through  space. 

Dust  Particles.  —  Even  more  minute  in  quantity  than 
either  of  the  gaseous  elements  of  the  air  is  the  solid  con- 
stituent. The  solid  particles  that  float  about  in  the  air 
are  commonly  known  as  dust;  and  when  a  beam  of  light 
enters  a  room,  the  larger  dust  particles  are  seen  dancing 
to  and  fro.  In  the  term  dust  are  included  many  differ- 
ent particles  which  are  so  light  that  they  may  float  in  the 
air.  Some  are  visible  to  the  eye ;  others,  and  the  majority, 
are  microscopic  in  size. 

When  wood  is  burned,  carbon  combines  with  oxygen  to 
form  carbonic  acid  gas ;  but  there  are  some  solid  particles 
which  do  not  become  transformed  to  gas.  Portions  of 
this  are  left  behind  as  ash,  while  some  rise  into  the  air 
and  float  away,  as  we  may  see  by  watching  the  smoke 
rising  from  a  chimney.^  In  large  cities  so  much  smoke 
is  sent  into  the  air,  that  a  dull  cloud  hovers  over  them, 
and  the  sun  shines  less  intensely  than  in  the  open  coun- 
try. Dust  is  also  blown  into  the  air  from  the  ground,  and 
there  are  many  microscopic  microbes,  and  quantities  of  tiny 
solid  substances  of  various  kinds. 

1  That  solid  particles  are  rising  from  even  the  blaze  of  a  candle  may  be 
proved  by  holding  a  piece  of  glass  over  the  flame  and  watching  it  become 
covered  with  soot. 


GENERAL  FEATURES   OF  THE  AIR  39 

So  much  dust  comes  from  these  various  sources,  that  if 
allowed  to  accumulate,  the  air  would  in  time  become  so 
impure  that  the  sun's  rays  would  be  obscured,  perhaps 
even  more  than  they  are  in  the  large  cities,  on  days  when 
the  air  is  quiet ;  but  the  dust  is  constantly  being  removed, 
some  by  slowly  settling  to  the  earth,  some  by  the  action 
of  rain,  which  as  it  descends  through  the  air,  catches  and 
carries  it  to  the  ground.  So  the  rain  purifies  and  fresh- 
ens the  atmosphere;  and  if  a  raindrop  is  examined  under 
a  powerful  microscope,  it  is  found  to  contain  many  minute 
solid  bits. 

The  amount  of  dust  varies  greatly;  at  times  the  air  is 
clear  and  free  from  these  impurities,  and  then  the  sun 
shines  brightly  and  the  sky  has  a  beautiful  blue  tint. 
Again,  particularly  during  drouths,  when  forest  fires  are 
common,  and  rains  have  not  come  to  remove  the  solids, 
the  air  is  hazy  and  the  sky  and  sun  dull,  sometimes  almost 
obscured.  At  such  a  time  the  raindrops  of  a  slight 
shower  contain  enough  dust  to  discolor  white  paper. 
Over  some  cities  where  soft  coal  is  burned,  the  dust  of 
the  air  settles  in  sufficient  quantities  to  discolor  white 
objects. 

Although  near  cities  there  is  more  dust  in  the  air  than  elsewhere, 
there  are  tiuies  in  desert  regions  when  sand  particles,  even  of  consid- 
erable size,  are  whirled  into  the  air  by  the  wind,  and  kept  there  by  its 
motion,  producing  sand  storms  which  shut  out  from  view  even  the 
objects  close  at  hand.  These  days  are  exceptional,  and  the  sand  soon 
settles  when  the  air  again  becomes  quiet.  A  violent  volcanic  erup- 
tion also  causes  dust  to  spread  high  into  the  air,  and  this  at  times 
travels  for  thousands  of  miles  before  settling  to  the  earth,  while  near 
the  volcano  the  darkness  of  night  may  be  produced  at  midday.  Over 
the  ocean  there  is  less  dust  than  over  the  land,  and  high  in  the  atmos- 
phere, and  on  mountain  peaks,  the  air  is  purer  than  on  the  lowlands. 


40  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Dust  particles  are  of  much  importance  in  the  action  of 
the  air.  The  microbes  which  are  present,  spread  disease. 
The  solid  particles  appear  to  serve  as  nuclei  around  which 
vapor  condenses  to  form  fog  particles  and  rain ;  and  their 
presence  in  the  atmosphere  is  responsible  for  many  of  the 
effects  of  sky  and  cloud  color  with  which  we  are  familiaj^^.-- 

Height  of  the  Atmosphere.  —  It  is  not  to  be  supposed 
that  the  upper  limits  of  the  air  are  sharply  defined,  like 


Fig.  17. 


To  illustrate  the  decrease  in  density  of  the  air  from  sea  level  to 
the  higher  regions. 

the  surface  of  the  liquid  ocean.  So  far  as  we  know,  the 
atmosphere  becomes  less  and  less  dense  as  the  distance 
from  the  ground  increases,  and  probably  gradually  fades 
away,  until  there  is  an  almost  indefinite  boundary  separat- 
ing it  from  the  great  void  of  space.  No  one  has  ever  been 
near  this  limit,  and  so  we  can  only  conjecture  as  to  its 
nature;  but  that  there  is  some  such  boundary  between 
empty  space  and  terrestrial  atmosphere,  is  proved  by  the 
behavior  of  meteors  and  shooting  stars. 

These  wanderers  in  space,  though  sometimes  large,  are 


GENERAL  FEATURES   OF  THE  AIR  41 

usually  tiny  particles,  which,  like  the  earth,  are  moving 
in  an  orbit  around  the  sun,  travelling  also  with  terrific 
velocities.  When  in  space  they  are  cold,  and  to  us  invis- 
ible; but  when  they  cross  the  path  of  the  earth,  and 
encounter  the  gases  of  the  atmosphere,  the  friction  causes 
heat,  just  as  heat  is  generated  when  a  knife  is  held  upon 
a  dry  grindstone  that  is  revolving.  The  meteors  then  begin 
to  glow,  and  in  most  cases  to  finally  burn  up  and  dis- 
appear. They  flash  out  suddenly  as  brilliant  beams  of 
light,  and  leave  behind  a  track  of  fire,  which  itself  quickly 
disappears.  This  furnishes  proof  that  the  meteors  come 
from  a  place  where  nothing  impedes  their  passage,  to  one 
where  they  encounter  resistance,  which,  though  only  that 
caused  by  an  invisible  gas,  nevertheless  serves  to  destroy 
all  but  the  largest,  which  sometimes  fall  to  the  earth. 

In  balloons,  and  on  mountain  tops,  men  have  ascended 
to  a  height  of  five  or  six  miles  above  the  level  of  the  sea. 
Air  is  still  found,  but  it  is  so  light  and  rarefied  that 
breathing  is  difficult,  and  such  ascents  are  sometimes 
dangerous  even  to  life.  The  air  has  definite  weight, 
which  can  be  measured  (Chapter  VII) ;  and  from  meas- 
urements at  various  heights,  we  know  that  more  than  one- 
half  of  its  whole  bulk  rests  within  four  miles  of  the  earth's 
surface,  and  fully  two-thirds  within  the  lower  six  miles 
(Fig.  17).  But  while  so  large  a  percentage  of  its  total 
bulk  is  near  the  earth,  the  atmosphere  is  not  limited  to  so 
shallow  a  depth.  As  the  elevation  becomes  greater,  the 
molecules  of  gas  become  further  and  further  apart,  and 
the  air  less  dense.  The  measurements  of  the  height  at 
which  shooting  stars  begin  to  glow,  seem  to  prove  that 
there  is  some  air  at  an  elevation  of  fully  500  miles  from 
the  surface  (Fig.  4). 


42  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Changes  in  the  Air.  —  Already  it  has  been  hinted  that 
the  atmosphere  is  elastic  and  mobile,  and  that  there  are 
many  changes  in  progress.  The  heat  from  the  sun  warms 
the  earth  and  air,  heating  some  parts  more  than  others, 
warming  by  day,  and  allowing  cooling  at  night,  causing 
intense  warmth  in  some  seasons  and  allowing  the  opposite 
season  to  be  cool  or  cold.  So  the  temperature  varies  from 
one  point  to  another,  and  from  day  to  day,  as  well  as  from 
season  to  season.  Since  the  air  is  elastic  and  easily  dis- 
turbed, these  differences  in  warmth  cause  movements; 
and  so  the  air  is  in  constant  motion,  now  violent,  now 
moderate  (Chapter  VII). 

Water  evaporates  from  the  land  and  the  seas,  and  the 
changes  in  temperature  and  movement  of  the  air  cause 
dry  winds  to-day,  and  possibly  damp  winds  to-morrow. 
The  vapor  condenses,  forming  dew,  or  possibly  clouds; 
and  even  storms  may  develop,  moving  across  the  land  and 
causing  rain  to  fall.  So  the  air  is  ever  variable,  and  no 
two  successive  days  are  exactly  alike.  The  weather  of 
a  place  near  the  seashore  is  different  from  that  in  the 
interior  of  the  continents ;  of  the  equatorial  regions,  from 
that  of  the  higher  latitudes ;  of  the  mountain  top,  from  the 
plain.  Infinite  variety  is  thus  introduced,  and  while  it 
will  be  impossible  to  state  all  of  these  differences,  in  the 
next  few  chapters  we  will  point  out  some  of  the  principles 
upon  which  they  depend,  and  illustrate  them  by  a  few 
examples. 

1/ 


CHAPTER   IV 

light,  electricity,  and  magnetism 

Light 

Nature  of  Light. — When  a  bar  of  iron  is  placed  in  a 
fire,  it  becomes  hot,  and  soon  begins  to  glow;  its  black 
color  is  lost,  and  it  becomes  red  or  even  white  hot.  It 
then  gives  out  light  and  heat,  and  we  are  able  either  to 
read  by  its  light,  or  to  warm  our  hands  by  holding  them 
over  it.  Like  the  iron,  the  sun  is  a  very  hot  body  which 
shines  with  a  fiery  light. 

If  we  place  a  white-hot  bar  of  iron  at  the  end  of  a  room, 
it  can  be  seen  from  the  opposite  side,  and  we  may  even 
be  able  to  see  it  at  a  distance  of  half  a  mile.  Something 
comes  from  the  iron,  which  upon  reaching  our  eyes,  pro- 
duces there  the  sensation  of  light.  The  hypothesis  which 
physicists  have  for  explaining  this,  is  the  undulatory  theory 
of  lights  according  to  which  it  is  believed,  that  a  series  of 
undulations,  or  waves,  are  started  in  an  invisible,  and  to 
us  entirely  mysterious  substance,  called  ether^  which  per- 
vades all  space.  These  waves  are  thought  to  be  somewhat 
like  those  in  water,  but  they  travel  at  the  almost  incred- 
ibly rapid  rate  of  about  186,000  miles  a  second.  So  if  a 
lamp  is  lighted  at  a  distance  of  a  mile,  we  perceive  it 
almost  at  the  same  instant. 

43 


44  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

The  hot  solar  orb  is  at  all  times  emitting  this  radiant 
energy/  (light  and  heat)  into  space  in  all  directions.  So 
rapid  is  the  movement  of  the  rays,  that  the  light  and  heat 
travel  across  space  to  the  earth,  over  a  path  of  more  than 
92,800,000  miles,  in  a  little  over  8  minutes.  Only  a  very 
small  part  of  the  light  and  heat  from  the  sun  reaches  us 
(Fig.  12)  and  most  of  it  goes  out  into  space,  where,  so  far 
as  we  know,  it  is  lost.^  Just  as  warmth  and  light  from 
the  hot  bar  of  iron  diminish  as  the  distance  from  it  is 
increased,  so  the  heat  and  light  of  the  sun  lose  intensity, 
until  at  the  planet  Neptune  their  amount  must  be  slight. 

According  to  this  theory,  light  is  not  a  simple  wave,  but 
a  complex  series  moving  with  slightly  different  wave 
lengths,  and  upon  reaching  the  eye  they  together  produce 
the  phenomenon  which  we  call  white  Ught.^  Sometimes, 
and  by  various  causes,  this  white  light  has  some  of  its 
waves  removed,  and  then  we  obtain  color.  So  while  light 
is  ordinarily  white,  the  sky  is  blue,  the  sunsets  red  or 
yellow,  the  leaves  of  trees  green,  and  the  flowers  vari- 
colored. The  difference  in  color  depends  upon  various 
conditions,  some  of  which  may  be  simply  stated,  while 
others  cannot  be  discussed  in  this  book. 

Reflection.  —  During  a  part  of  the  month  the  moon 
shines  brightly  in  the  heavens  with  a  silvery  white  light, 
and  again  it  disappears;  when  the  moon  is  young  or  old, 
we  see  the  bright  crescent,  but  the  dull  remainder  of  the 

1  It  is  somewhat  as  if  we  compared  a  lamp  in  a  room  to  the  sun,  and  a 
speck  of  dust  on  the  wall  to  the  earth, 

2  White  light  is  made  of  a  number  of  waves  each  having  a  color  of  its 
own.  We  recognize  onlj"  seven  of  these,  called  primary  colors,  —  violet, 
indigo,  blue,  green,  yellow,  orange,  and  red,  — the  colors  of  the  spectrum, 
which  we  see  in  a  rainbow.  This  may  be  illustrated  by  a  prism,  which 
breaks  up  the  white  light  into  its  component  colors. 


LIGHT  45 

sphere  does  not  shine  brightly.  Like  the  earth,  the  moon 
intercepts  some  of  the  sun's  rays,  and  at  times  these  are 
reflected  to  the  earth,  just  as  we  may  cause  a  sunbeam  to 
be  reflected  from  a  mirror  to  some  place  which  the  sun's 
rays  do  not  reach  directly.  On  the  moon  the  earth  appears 
as  a  brightly  illuminated  orb  in  the  sky. 

Gases  do  not  readily  reflect,  but  instead,  allow  the  light 
to  pass  easily  through  them,  being  transparent ;  but  a 
water  surface  does  reflect  readily,  and  as  the  sun's  rays 
reach  the  surface  of  a  water  body,  we  find  them  reflected 
from  this  in  the  form  of  a  dazzling  beam  of  light.  ^  So, 
too,  when  the  sun's  rays  reach  a  dense  cloud  bank,  the 
water  or  snow  particles  which  compose  the  cloud  may 
reflect  the  sunlight ;  and  while  the  great  mass  of  the  cloud 
appears  black  or  leaden  gray,  the  edge  upon  which  the 
sunbeams  strike,  becomes  transformed  to  dazzling  white. 
If  the  sunlight  is  colored,  because  of  some  change  in  the 
light  rays,  such  as  those  explained  below,  the  reflected 
light  may  he  colored. 

Pure  air  of  uniform  density  does  not  of  itself  reflect  the 
light;  but  when  a  beam  of  sunlight  passes  through  a  gap 
in  a  cloud,  its  path  to  the  earth  may  be  seen,  and  the  sun 
is  then  said  to  be  ''^drawing  ivater^  This  appearance 
must  be  the  result  of  reflection  of  light,  for  otherwise  the 
distant  rays  would  not  be  visible.  If  the  air  did  not 
reflect  light,  the  illumination  of  the  earth  in  the  daytime 
would  be  very  different.  The  places  in  shadow  would  be 
very  dark,  if  light  were  not  reflected  to  them  from  the  air 
and  from  the  land.  By  means  of  a  mirror,  we  may  reflect 
enough   sunlight   upon  a  shadow  to  entirely  destroy  it; 

1  By  an  experiment  with  a  dish  of  water,  or  a  mirror,  placed  in  the 
sunlight,  the  teacher  can  make  the  subject  of  reflection  plain. 


46  FIRST  BOOK  OF  PHYSICAL   GEOGBAPHT 

and  in  a  less  complete  way  the  natural  reflection  from 
many  surrounding  objects  is  engaged  in  the  partial 
destruction  of  shadows. 

Shadows  are  less  intense  on  hazy  than  on  very  clear 
days ;  for  then  there  are  many  solid  particles  in  the  air ; 
and  it  is  these  dust  particles^  rather  than  the  air  itself, 
which  cause  the  reflection  which  is  seen  when  the  sun 
is  "drawing  water."  In  addition  to  the  reflection  from 
the  dust,  there  is  sometimes  reflection  from  the  air  itself. 
This  is  because  the  atmosphere  varies  in  density.  We 
may  see  this  on  any  hot,  close  summer  day,  when  walking 
along  a  road  or  on  a  railway  track.  The  Avarm  air  is 
set  in  motion,  and  the  little  currents  thus  started  differ 
in  density,  and  as  they  reflect  light  from  their  surface  we 
see  them  dancing  about.  Each  different  layer  acts  some- 
what like  a  mirror. 

It  is  upon  this  principle  that  the  mirage  is  caused. 
The  traveller  on  the  desert  often  fancies  that  he  sees  a 
sheet  of  water  when  he  looks  down  upon  the  air  that  is 
warm-  near  the  surface,  so  that  it  has  different  density 
from  that  above,  causing  it  to  reflect  light.  On  a  lake  or 
seashore  the  mirage  lifts  the  distant  shores  above  the  water 
level,  and  ships  ma}-  be  seen  apparently  sailing  in  the  air. 
When  there  is  a  warm  layer  of  air  above  the  surface,  reflec- 
tions may  be  caused  from  it,  and  objects  may  then  appear 
inverted,  so  that  a  ship  may  sometimes  be  seen  apparently 
sailing  in  the  heavens  with  its  masts  pointed  toward  the 
earth. 

In  the  Arctic  regions,  during  the  summer,  the  mirage  is  very  com- 
mon ;  and  when  sailing  in  the  midst  of  floating  ice,  the  effect  of  the 
mirage  in  raising  the  ice  floe  above  the  water  level,  often  transforms 
the  broken  ice  surface  into  a  marvellously  complex  and  beautiful 


LIGHT  47 

series  of  white  imitations  of  cities,  castles,  and  turrets.  A  single 
piece  of  ice  is  sometimes  duplicated,  until  four  or  five  pieces  appear 
one  above  the  other.  Sometimes  these  join,  forming  a  column;  or, 
when  partially  joined,  a  sculptured  turret;  and  as  one  looks  upon 
such  a  scene,  there  is  constant  variation,  and  no  two  times  is  the  same 
view  seen  in  the  same  direction. ,  / 

Absorption.  —  Light  rays  pass  easily  through  some  sub- 
stances, which  are  then  said  to  be  trans'parent.  Nearly 
all  gases  are  transparent,  or  nearly  so  (then  called  trans- 
lucent)^ and  many  liquids  also  allow  light  to  pass  easily 
through  them ;  but  solid  substances  are  more  rarely  trans- 
parent. An  instance  of  a  transparent  solid  is  glass,  the 
transparency  of  which  serves  us  so  well  in  our  windows. 

When  light  encounters  bodies,  even  those  that  are  most 
transparent  absorb  some  of  the  light,  while  the  remainder  is 
either  reflected  or  allowed  to  pass  on  into  the  substance. 
Those  solids  and  liquids  which  do  not  allow  light  to  pass 
easily  through,  and  which  are  then  called  opaque,  absorb 
most  of  that  which  encounters  them.  When  very  little 
of  the  light  is  absorbed,  the  object  is  white  in  color;  when 
nearly  all  is  absorbed,  it  is  black.  But  as  white  light  is  a 
complex  of  many  waves,  each  producing  separate  colors,  it 
happens  that  many  objects  allow  some  of  the  rays  to  pass, 
while  others  are  reflected,  and  then  the  sensation  of  color 
is  produced.  If  green  is  reflected  in  excess  of  the  others, 
the  color  is  green ;  if  more  red  is  reflected  than  others,  a 
red  is  produced,  etc. 

This  absorption  of  sunlight  is  important  in  the  economy 
of  life,  particularly  of  many  forms  of  plant  life.  While 
a  potato  will  sprout  and  grow  in  a  cellar,  it  does  not  pro- 
duce fresh  green  leaves,  but  instead,  a  sickly,  yellowish- 
green  stalk  and  leaves.  Even  men  who  dwell  away  from 
the  sunlight  lose  their  freshness. 


48  FIRST  BOOK  OF  PHYSICAL   GEOGRAPnY 

Selective  Scattering.  —  The  light  of  the  sun  is  probably 
bluish  when  it  enters  the  upper  layers  of  the  atmosphere, 
becoming  white  in  its  passage  through  the  air.  In  this 
passage^  light  rays  suffer  much  change,  a  change  which  is 
not  at  all  times  the  same.  Sometimes  the  sky  is  a  deep 
azure  blue,  again  it  is  a  pale,  almost  colorless  blue,  and 
there  have  been  times  when  its  color  was  a  brassy  yellow. 

Light  waves,  which  are  of  various  lengths,  when  pass- 
ing through  air  that  is  impure,  find  their  progress  par- 
tially checked.  The  coarser  waves  of  yellow  and  red 
light  are  less  easily  disturbed  than  those  having  a  small 
wave  length,  like  the  violet  and  blue.  This  may  be  com- 
pared to  the  ripples  on  a  lake,  which  may  be  checked  in 
their  motion  by  a  small  sand  spit,  while  the  larger  storm 
waves  break  over  the  obstacle. 

By  this  interference^  some  of  the  rays  are  turned  to  one 
side  and  scattered.  Since  the  waves  that  have  small  wave 
length  are  more  easily  turned  aside,  the  violets  and  blues 
of  the  white  light  are  scattered  even  if  the  air  is  very 
clear.  Hence  the  intensity  of  the  blueness  of  the  sky  is 
greater  on  particularly  clear  than  on  hazy  days,  when  the 
scattered  blue  rays  are  partially  obscured  by  the  scattering 
of  the  other  and  coarser  rays.  When  there  is  much  dust 
in  the  air,  even  the  yellow  may  be  scattered ;  and  these, 
being  then  more  intense  than  the  blue,  give  to  the  sky  a 
yellowish  tinge.  The  color  of  the  sky  therefore  depends 
upon  ivhich  of  the  rays  are  scattered;  and  since  certain 
waves  are  selected^  according  to  the  obstacle  encountered, 
the  process  is  called  selective  scattering,  y 

Refraction.  — While  there  are  several  other  peculiarities 
of  light,  some  of  which  are  important,  only  one  more  can 
be  easily  explained  in  this  book.     When  a  stick  is  placed 


LIGHT  49 

in  a  quiet  body  of  water,  with  a  part  extending  above  the 
surface,  it  appears  broken  at  the  water  surface.^  This  is 
due  to  refraction,  the  light  ray  itself  being  bent  as  it 
passes  into  the  denser  medium.  If  we  could  look  from 
the  water  to  the  air,  the  stick  would  still  appear  broken, 
but  would  incline  in  the  opposite  direction. 

This  bending  of  the  rays  affects  those  colors  which  have 
the  shorter  wave  lengths,  in  a  different  way  from  those 
with  the  longer  wave  lengths.  So  if  we  allow  a  sunbeam 
to  pass  through  a  glass  prism,  refraction  bends  the  rays 
and  affects  the  various  colors  differently.  Hence,  when  a 
light  ray  emerges  from  a  prism,  instead  of  all  the  rays 
combining  to  cause  white,  the  colors  of  the  spectrum  are 
thrown  upon  the  floor,  and  we  are  then  able  to  recog- 
nize the  seven  primary  colors  mentioned  above.  Many  of 
our  atmospheric  colors,  especially  those  of  sunset,  depend 
in  part  upon  this  principle  of  refraction  of  light  rays,  in 
passing  through  substances  of  different  density. 

The  Colors  of  Sunrise  and  Sunset.  —  When  the  sun  is 
setting,  its  brilliancy  is  usually  so  decreased  that  we  may 
look  directly  at  it ;  and  if  the  air  is  dusty,  it  may  be  a  great 
red  orb,  because  the  more  delicate  light  waves  have  been 
scattered  in  passing  through  the  great  thickness  of  dust- 
filled  air  (Fig.  21).  As  the  sun  disappears,  a  glow  of 
yellow,  or  red,  overspreads  the  sky  in  the  west,  because, 
in  addition  to  the  blue  light  waves,  the  coarser  reds  and 
yellows  have  been  scattered  in  passing  through  the  mote- 
filled  air.  Those  rays  which  come  from  near  the  horizon 
are  most  destroyed,  and  hence  the  coarsest  of  all  prevail 
there,  giving  red  colors,  while  those  above  the  horizon, 
passing  through  less  air,  are  yellow. 

1  This  is  an  experiment  which  any  pupil  can  try  for  himself. 


50  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

These  solar  rays  are  bent,  or  refracted,  in  passing  through 
the  layers  of  air,  and  when  we  see  the  sun  just  beginning 
to  descend  behind  the  hills,  it  is  really  below  the  horizon. 
Selective  scattering  lengthens  the  zone  of  coloring,  so  that 
the  sunset  colors  often  extend  far  on  either  side  of  the  set- 
ting sun.  In  a  clear  sky  the  colors  of  sunset  are  arranged 
in  a  semicircular  series,  with  the  sun  near  the  centre.  At 
first  the  colors  are  yellow,  fading  out  through  tints  of  green 
to  the  sky-blue  above.  The  yellow  changes  to  red  near  the 
horizon.  A  second  fainter  series  of  colors,  the  afterglow^ 
often  illuminates  the  sky,  after  the  first  glow  of  sunset 
has  faded.  In  reverse  order  the  sunrise  colors  exhibit  the 
same  changes. 

At  sunset  there  is  a  delicate  pink  tint  in  the  eastern  sky,  grading 
upward  and  downward  into  the  blue,  forming  an  arch,  known  as  the 
twilight  arch.  This  is  the  result  of  reflection  of  some  of  the  scattered 
rays  produced  at  sunset ;  and  the  dark  or  blue  color,  below  the  re- 
flected pink,  is  the  shadow  of  the  earth  cast  against  the  sky. 

These  are  some  of  the  normal  effects  of  the  setting  sun 
in  clear  weather.  An  increase  of  dust  in  th.e  air,  at  first 
increases  the  intensity  of  the  coloring;  but  beyond  a  cer- 
tain point,  the  dust  deadens  the  colors,  so  that  in  a  very 
hazy  sky  the  sunset  colors  are  absent,  because  even  the 
waves  of  red  and  yellow  light  are  so  scattered  that  they 
do  not  reach  the  eye.  Very  often,  both  at  sunset  and 
sunrise,  the  horizon  is  partly  occupied  by  clouds,  and  the 
cloud  particles  reflect  and  refract  the  light  rays,  produc- 
ing a  marvellously  beautiful  and  varied  series  of  shades 
and  tints  of  reds,  yellows,  brilliant  gold,  and  deep  purple. 
Our  most perfeet  sunsets  are  in  a  clear  sky;  but  the  most 
beautiful  come  when  the  heavens  are  partly  clouded.         . 


LIGHT  5i 

The  Rainbow.  —  Standing  at  the  foot  of  Niagara  Falls, 
one  may  often  see  a  beautiful  rainbow  outlined  in  the 
spray  that  dashes  into  the  air  at  the  base  of  the  mighty 
cataract.  Though  forming  an  arc  of  a  smaller  circle  than 
that  seen  in  the  eastern  sky  after  a  summer  thunder  storm, 
it  is  the  same  in  cause.  In  each  case  there  are  drops  of 
water  —  spray  in  Niagara,  and  raindrops  in  the  rainbow  — 
through  which  the  rays  of  the  sun  are  passing;  and  as  the 
rays  enter  and  emerge  from  the  water  drops,  they  are 
refracted,  just  as  in  the  case  of  the  light  passing  through 
a  prism  of  glass.  This  refraction  separates  the  rays  into 
the  rainbow  colors,  and  these  are  sent  back  to  the  eye  by 
reflection  from  the  drops  of  rain. 

The  form  of  the  rainbow  is  that  of  a  segment  of  a  circle, 
with  the  ends  resting  on  the  horizon ;  and  the  extent  of 
the  arc  depends  upon  the  position  of  the  sun,  being  small 
when  the  sun  is  high  in -the  heavens,  and  nearly  a  semi- 
circle when  it  is  near  the  horizon.  The  colors  of  the 
rainbow  are  those  of  the  spectrum,  with  the  red  outside, 
and  at  times  a  second  bow  is  produced  above  the  true 
rainbow.  In  this  the  red  is  on  the  inside.  Each  person 
sees  a  different  rainbow;  but  all  see  the  same  general 
features,  because  the  raindrops  always  act  in  the  same 
way  in  refracting  light. 

Halos  and  Coronas.  —  There  are  other  peculiar  and  exceptional  effects 
of  light,  only  two  of  which  will  be  briefly  mentioned.  The  halo, 
or  ring  around  the  sun  or  moon,  occurs  when  the  upper  air  is  over- 
spread with  nearly  transparent  clouds,  composed  of  particles  of  ice. 
Usually  the  halo  is  a  ring  of  white  light;  but  when  best  developed  it 
has  the  colors  of  the  spectrum,  with  the  red  inside.  Arctic  explorers 
describe  brilliant  halos,  for  in  these  cold  regions,  the  air  often  bears 
numerous  ice  crystals,  and  it  is  refraction  of  light  passing  through 
these,  and  reflected  from  their  surface,  that  produces  the  halos. 


52  FIRST  BOOK  OF  PHYSICAL   GFOGRAPHT 

When  denser  clouds  partly  obscure  the  sun,  the  interference  of 
these  with  the  rays  of  light,  sometimes  produces  coronas,  which 
are  circles  of  colored  light  concentrically  arranged,  and  usually 
of  small  diameter,  with  the  red  on  the  outside.  At  other  times  a 
Bar  of  light  sometimes  extends  from  the  sun,  and  at  times  a  cross 
is  formed  by  two  such  bars.  In  rare  cases  these  bars  occur  at  the 
same  time  with  coronas,  and  the  circles  are  then  divided  into  foui 
segments. 

Sunlight  Measurement.  —  Physicists  have  measured  the  velocity  of 
light  by  various  means ;  and  nearly  all  the  phenomena  of  light  have 
received  a  satisfactory  explanation  on  the  basis  of  the  undulatory 
theory.  With  these  measurements  we  are  not  concerned;  but  meteo- 
rologists sometimes  study  the  intensity  and  duration  of  sunlight.  The 
former  may  be  obtained  by  means  of  the  black  bulb  thermometer  (p.  60), 
the  latter  by  the  sunshine  recorder.  This  is  a  metal  box  so  placed 
that  from  sunrise  to  sunset  the  sunlight  shines  into  it  through  a  hole. 
On  the  inside  of  the  box  a  piece  of  photographic  (or  blue  print) 
paper  is  placed,  and  the  sunbeam,  entering  the  hole,  travels  over  this, 
thus  marking  its  presence  by  a  line  that  is  continuous  if  the  sun 
shines  all  day,  and  broken  if  interrupted  by  clouds.  At  night  the 
photographic  paper  is  taken  out  and  developed,  and  thus  a  line  is 
marked  where  the  sun  shone,  while  no  line  is  present  if  the  sun's 
rays  were  interrupted.  A  similar  result  may  be  obtained  from  the 
black  bulb  thermometer. 

By  such  means  we  learn  that  in  1892  the  sun  at  Yuma,  Arizona, 
shone  for  fully  80  %  of  all  the  time  that  it  stood  above  the  horizon ; 
at  San  Diego,  California,  62%;  at  Salt  Lake  City,  Utah,  57%;  at 
Washington,  D.C.,  52%;  at  St.  Louis,  Missouri,  44%;  at  Eastport, 
Maine,  44%;  at  Buffalo,  N.Y.,  40%. 

V 

Electricity  and  Magnetism 

Lightning.  —  During  •  thunder  storms,  and  other  violent 
disturbances  of  the  air,  an  electric  spark  is  often  caused  to 
pass  from  cloud  to  cloud,  or  from  a  cloud  to  the  ground. 
Lightning  is  then  produced,  and  the  sound  caused  by  the 


ELECTRICITY  AND  MAGNETISM  53 

passage,^  as  it  echoes  and  reverberates  among  the  clouds, 
causes  the  roar  and  crash  of  thunder.  When  thunder 
storms  are  at  a  distance,  and  often  when  they  are  below 
the  horizon,  a  flash  of  lightning  illuminates  the  distant 
sky,  and  we  see  heat  lightning.  It  is  possible  that  atmos- 
pheric electricity  has  some  influence  upon  the  formation 
of  rain,  though  of  this  there  is  some  doubt ;  and  although 
producing  some  vivid  effects  in  the  form  of  lightning,  it 
is  not  now  recognized  as  an  important  feature  of  the  air.^ 

Magnetism.  —  Every  one  is  familiar  with  the  common 
magnet,  which  is  a  magnetized  piece  of  iron  capable  of 
attracting  other  particles  of  iron.  The  earth  is  a  great 
magnet  with  two  poles  of  attraction,  one  soath  of  Aus- 
tralia, in  the  Antarctic  region,  the  other  on  Boothia 
Island,  north  of  Hudson's  Bay,  in  the  Arctic.  These 
poles  attract  the  needle  of  the  compass,  which  is  a  piece 
of  magnetized  iron,  so  that  the  north  end  of  the  needle 
points  toward  the  north  magnetic  pole.  The  attractive 
force  is  beneath  the  earth's  surface,  so  that  near  the  pole 
the  magnetic  needle  dips  vertically  toward  the  ground. 
This  condition  of  terrestrial  magnetism  is  exceedingly 
important,  for  it  furnishes  us  an  easy  means  of  obtain- 
ing directions  by  compass.  This  subject  calls  for  con- 
stant study,  because  the  attractive  force  steadily  varies, 
so  that  the  pole  is  not  always  in  the  same  place. 

Magnetic  action  is  also  present  in  the  sun  ;  and  on  the 
earth  we  are  able  to  detect  this  by  means  of  delicate  instru- 
ments.    Sometimes  this  solar  magnetism  produces  what 

1  By  an  electric  spark  from  Ley.den  jars  an  imitation  of  lightning  and 
thunder  may  be  produced.    An  electric  car  furnishes  frenuent  ilkistrations. 

2  Just  how  this  electricity  is  generated,  and  why  it  appears,  is  not 
exactly  understood. 


5i  FIBST  BOOK  OF  PHYSICAL   GEOGRAPHY 

are  known  as  magnetic  storms,  when  electric  apparatus  is 
disturbed  and  the  atmosphere  seems  to  be  under  the  influ- 
ence of  magnetic  action.  There  is  some  relation  between 
solar  magnetism  and  sun  spots. 

At  times  the  northern  sky  may  become  illuminated  at 
night,  by  the  strange  light  known  as  the  Northern  Lights, 
or  Aurora  Borealis,  This  is  appai-ently  some  magnetic 
disturbance  in  the  air,  by  which  a  faint  light  is  produced. 
Usually  colorless,  the  aurora  sometimes  assumes  various 
tints,  and  a  variety  of  form  that  at  times  is  remarkable, 
now  waving  like  a  drapery,  now  shooting  backward  and 
forward  with  great  rapidity,  while  always  it  is  so  dim 
that  the  stars  shine  through  it.  The  Northern  Lights 
are  much  more  frequently  and  better  developed  near  the 
magnetic  pole  than  elsewhere,  although  they  may  often 
be  seen  in  the  United  States.  In  some  way  this  appears 
to  be  related  to  the  magnetism  of  the  earth  itself. 

Although  careful  studies  have  been  carried  on  for  a 
long  time,  the  question  of  magnetism  is  still  far  from 
settled;  and  when  it  is  understood,  the  explanation  may 
throw  much  light  upon  various  questions.  There  is 
reason  for  believing  that  the  north  magnetic  pole  of  the 
earth,  and  the  magnetism  of  the  sun,  are  among  the  causes 
which  produce,  or  at  least  direct,  the  great  rain  storms 
which  travel  across  the  country,  causing  most  of  the  rain 
that  falls  in  the  northern  states.  Here  is  one  of  the  great 
unsolved  problems  with  which  Nature  confronts  us,  and 
yet  one  whose  influence  is  constantly  present  and  impor- 
tant. Our  sailors,  surveyors,  and  map  makers  are  making 
use  of  a  force  whose  nature  no  one  understands. 


CHAPTER   V 

SUN'S  HEATi 

Nature  of  Heat.  —  Like  light,  our  supply  of  heat  comes 
directly  from  the  sun,  whence  it  is  emitted  in  company 
with  light. 

Some  hot  bodies,  like  a  stove,  do  not  usually  produce 
the  sensation  of  light  on  the  eye,  but  if  their  heat  is 
increased,  light  is  finally  produced.  Bodies  which  reflect 
sunlight,  such  as  a  piece  of  white  paper,  do  not  become 
warm  as  quickly  as  those  like  black  paper,  which  do  not 
reflect  light.  However,  there  is  an  intimate  connection 
between  these  phenomena  of  radiant  energy  (light  and 
radiant  heat),  which  have  the  same  origin  and  differ  only 
in  the  size  of  the  wave. 

Radiant  heat,  like  light,  travels  to  us  across  space  as  a 
series  of  waves,  moving  with  exceedingly  rapid  vibrations. 
It  is  the  great  life  giver,  for  it  warms  our  sphere,  and  upon 
its  presence  all  forms  of  life  depend  for  existence.  Like 
light,  heat  changes  its  behavior  on  reaching  the  earth,  and 
therefore,  for  an  understanding  of  the  distribution  of  solar 
heat  over  the  globe,  we  must  iirst  learn  something  of  its 
peculiarities. 

Reflection  of  Heat.  —  Most  bodies  that  allow  light  to 
pass  easily  through  them,  offer  as  little  resistance  to  heat. 

1  It  is  exceedingly  important  that  the  students  thoroughly  grasp  the 
principles  treated  in  this  chapter. 

55 


56  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Hence  radiant  energy  (both  light  and  heat)  enters  and 
passes  readily  through  the  transparent  atmosphere;  and  if 
the  temperature  out  of  doors  is  zero,  a  thermometer  in  a 
room  placed  near  a  window,  in  the  direct  rays  of  the  sun, 
will  record  a  much  higher  temperature  than  that  of  the  air 
outside.  Such  bodies  as  air  and  glass,  which  are  trans- 
parent to  heat,  are  said  to  be  diathermanous. 

As  in  the  case  of  light,  all  bodies  can  reflect  some  heat, 
and  we  even  obtain  a  small  quantity  of  reflected  heat  from 
the  moon.  Smooth  bodies,  and  those  having  a  light  color, 
reflect  both  heat  and  light  better  than  others,  and  we  may 
prove  this  at  any  time  by  standing  in  a  shadow  upon  which 
the  light  and  heat  from  a  window  are  being  reflected. 
Quarrymen  working  in  pits  partly  enclosed  by  rock  walls, 
notice  the  same  thing  on  a  hot  summer  day,  when  the 
walls  of  the  quarry  reflect  heat,  and  add  to  that  which 
falls  directly  upon  them  from  the  sun.  It  is  partly  for 
this  reason  also,  that  the  streets  of  a  city  are  hotter  in 
summer  than  the  open  country;  for  then  not  only  does 
heat  fall  directly  upon  the  street,  but  some  is  reflected 
from  the  enclosing  walls  of  buildings.  So  also  we  may 
become  sunburned  in  summer,  even  though  our  face  is 
kept  in  shadow  by  means  of  a  broad  hat.  The  reflected 
heat  from  the  ground  performs  this  work;  and  since  a 
water  surface  reflects  better  than  the  ground,  one  becomes 
sunburned  much  more  quickly  when  on  the  water  than  on 
the  land. 

During  their  passage  through  the  air,  some  of  the  heat 
rays  are  reflected  and  scattered  by  the  dust  particles  that 
are  present;  but  most  of  them  pass  on  and  reach  the 
ground.  Most  of  the  light  immediately  escapes,  and 
when  the  sun's  rays  are  absent,  darkness  prevails;  but 


SUN'S  HEAT  57 

heat  in  part  remains,  and  its  effect  is  still  felt,  to  some 
extent,  even  when  the  sun  rises  in  the  morning.^  How- 
ever, a  large  percentage  of  the  heat  rays  that  reach  the 
earth,  pass  directly  away,  being  turned  back  by  reflection; 
and  so,  as  far  as  the  earth  is  concerned,  these  are  lost  in 
space. 

Absorption  of  Heat.  — A  portion  of  the  heat  is  absorbed. 
Black  bodies  absorb  so  much  of  the  sunlight  that  they 
return  to  the  eyes  no  distinct  color.  Such  bodies  also 
absorb  much  of  the  heat  that  conies  to  them,  and  hence  in 
summer  the  asphalt  pavement  of  cities,  or  the  black  rocks 
of  the  country,  become  hot.  If  we  place  a  piece  of  black 
cloth  upon  a  snow  bank,  in  such  a  position  a^s  to  be 
exposed  to  the  direct  rays  of  the  sun,  its  warmth,  result- 
ing from  absorption,  will  cause  the  snow  beneath  it  to 
melt,  and  the  cloth  will  sink  into  the  snow  even  during 
cold  weather;  but  if  a  white  cloth  is  used,  much  more  of 
the  radiant  energy  is  reflected,  and  the  cloth  does  not 
become  nearly  so  warm.  Hence  the  warming  effect  of  the 
sun's  rays  varies  greatly,  not  only  with  the  location,  but 
also  with  the  material  that  is  being  warmed.    (/ 

Radiation  of  Heat.  —  When  we  receive  heat  from  a  stove, 
rays  come  to  us  that  are  being  radiated  from  the  iron. 
The  heat  that  comes  from  the  sun  is  similar  radiant 
energy  which  this  great  body  is  emitting;  for  a  heated 
body  is  able  to  give  out  its  heat  to  cooler  surrounding 
areas,  until  the  temperature  of  the  two  is  equalized. 
Hence  the  sun  will  continue  to  radiate  heat  into  space 
in  all  directions,  until  its  temperature  is  reduced  to  that 

1  This  may  he  illustrated  in  this  way  :  if  the  sun  shines  into  a  room,  it 
becomes  light  and  also  is  warmed  ;  close  the  blinds  and  the  light  ceases, 
but  objects  that  were  reached  by  the  sunlight  will  still  remain  warm. 


68  FIRST  BOOK  or   PUTSICAL   GEOGRAPnT 

of  space:  just  as  a  stove  in  which  the  fire  has  gone 
out  will  continue  to  radiate  heat  into  the  room  until 
its  temperature  is  reduced  to  that  of  the  surrounding 
air. 

The  same  is  true  of  the  heat  which  comes  to  the  earth 
from  the  sun,  and  which  stays  upon  the  surface  for  awhile. 
Some  of  this  is  absorbed,  and  the  ground  is  warmed ;  but 
during  all  the  time  that  this  heat  is  coming  to  the  earth, 
it  is  being  sent  away  into  space,  some  by  reflection,  some 
by  direct  radiation  ;  and  it  passes  through  the  atmosphere 
in  a  way  similar  to  that  in  which  it  entered.  Even  dur- 
ing the  hottest  summer  days,  radiation  is  in  progress ;  but 
the  ground  continues  to  warm  through  the  day,  because 
more  heat  is  being  absorbed  than  can  be  radiated.  How- 
ever, as  soon  as  the  sun  descends  behind  the  western 
horizon,  the  supply  is  cut  off,  while  radiation  continues, 
and  hence  the  ground  becomes  cooler,  and  continues  to 
cool  until  the  sun  again  rises. 

During  the  summer  the  days  are  longer  than  the  nights, 
and  more  heat  comes  than  can  be  radiated.  Therefore  the 
ground  warms  day  after  day ;  but  in  the  winter,  radiation 
is  in  excess  of  the  supply,  so  that  the  ground  becomes 
cooler  and  cooler,  until  the  days  have  again  perceptibly 
lengthened.  In  this  way  the  earth  disposes  of  its  surplus 
heat,  and  hence  the  heat  of  one  year  is  not  greatly  dif- 
ferent from  that  of  the  preceding.  If  it  were  not  for 
this  action  of  radiation,  each  year  would  witness  an  in- 
crease of  heat ;  and  the  air  and  earth  would  soon  become 
intensely  hot  if  all  of  the  rays  were  not  sent  away. 

Radiation  is  extremely  important  in  explaining  many 
of  the  features  of  the  air,  and  in  later  pages  we  will 
need  to  refer  to  it  again  and  again.     Some  bodies  radiate 


SUN'S  HEAT  59 

much  better  than  others:  grass  and  leaves  are  better 
radiators  than  the  bare  ground,  and  the  earth  radiates 
more  readily  than  water.  Hence  the  sun's  rays  affect 
various  parts  of  the  earth  in  a  different  way,  and  here 
is  another  reason  for  variation  in  temperature  of  the 
earth's  surface. 

Conduction  of  Heat.  —  If  the  end  of  a  rod  of  copper,  or 
even  of  iron,  is  placed  in  the  fire,  the  end  in  contact  with 
the  fire  becomes  very  hot,  and  soon  the  other  end,  which 
is  entirely  away  from  the  source  of  heat,  itself  becomes 
warm,  and  after  awhile  so  hot  that  it  cannot  be  held. 
The  heat  of  one  end  passes  through  the  iron  by  conduction, 
being  transmitted  from  molecule  to  molecule.  In  the 
same  Avay,  the  rays  of  the  sun,  which  come  in  contact 
with  only  the  veri/  surface  of  the  land,  have  their  heat 
gradually  conducted  down  into  the  soil.  But  the  ground 
is  not  so  good  a  conductor  as  iron,  and  at  a  depth  of 
three  or  four  feet,  the  sun's  rays  produce  little  effect 
even  in  summer,  while  at  a  depth  of  ten  or  twenty  feet, 
the  influence  of  the  sun  is  almost  absent.  Thus  the 
sun  warms  only  the  veri/  surface  of  the  land.  Water 
and  air  are  even  poorer  conductors;  but  the  air  which 
rests  directly  on  the  ground,  is  gradually  warmed  by 
contact  with  the  warm  earth,  and  by  conduction  this 
heat  is  slowly  transmitted  into  the  air  to  a  slight  dis- 
tance above  the  ground. 

Convection.  —  When  a  kettle  of  water  is  placed  upon  a 
stove,  the  iron  bottom  is  warmed,  and  the.  heat  is  con- 
ducted from  the  iron  into  the  water.  Heat  causes  expan- 
sion, as  any  one  may  see  by  watching  a  blacksmith  put 
an  iron  tire  on  a  wheel.  The  iron  is  warmed  and  placed 
outside  the  wheel,  which  it  fits  very  loosely;  but  as  it 


60  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

cools  by  radiation  and  conduction,  it  contracts  and  binds 
the  wood  of  the  wheel  firmly  together.  ^ 

The  expansion  of  a  liquid  or  a  gas  makes  it  less  dense 
or  lighter.  Therefore  when  the  Avater  in  the  bottom  of 
the  kettle  is  warmed,  it  becomes  lighter  than  the  layers 
above.  This  is  an  unnatural  condition,  for  the  lightest 
things  float,  as  oil  or  wood  will  float  on  water.  As  the 
warming  proceeds,  the  light  layers  of  lower  Avater  are 
forced  to  rise  by  the  sinking  of  the  heavy  cold  layers, 
which  are  drawn  down  by  gravity.  This  causes  a  boiling 
or  convection.  In  the  same  way  the  lower  layers  of  the 
atmosphere  are  warmed  by  conduction  from  the  ground, 
which  has  absorbed  heat.  These  become  lighter,  and  an 
atmospheric  boiling  or  convection  is  inaugurated. 

Ordinarily  this  convection  is  quiet  and  unnoticeable; 
but  on  a  dusty  road,  or  a  railway  track,  during  a  hot 
summer  day,  the  boiling  of  the  air  may  be  actuallj^  seen, 
as  the  little  rising  currents  of  warm  air  reflect  light  to 
the  eye.  The  air  seems  to  be  in  violent  motion,  and 
sometimes  this  is  enough  to  obscure  objects  at  no  great 
distance.  By  this  process  of  convection  the  atmosphere 
is  set  in  motion  in  a  much  larger  way,  and  this  action 
furnishes  an  explanation  of  many  of  the  winds  of  the 
earth  (Chapter  VII). 

Heat  on  the  Land.  —  A  few  words  in  summary  will  show 
how  the  land  is  warmed  and  cooled.  During  the  daytime 
the  earth  absorbs  heat,  much  in  summer  and  relatively 
little  in  winter;  and  at  all  times  it  is  radiating^  though 

1  So  also  the  warming  of  the  rails  of  a  track  in  summer  causes  them  to 
expand,  so  that  the  joints  fit  together,  while  in  the  cold  winter  they  con- 
tract and  spread  apart.  Therefore  in  laying  rails  it  is  necessary  to  leave 
a  small  space  for  this  movement. 


sun's  heat  61 

at  night,  when  no  heat  comes,  the  effect  of  radiation  is 
most  pronounced.  Some  of  the  heat  is  conducted  below 
the  surface,  so  that  the  upper  layer  of  the  ground  is  less 
excessively  warmed  than  it  would  be  if  all  remained  where 
it  fell.  Moreover,  the  air  that  rests  upon  the  surface 
becomes  warm  by  conduction,  and  this  also  takes  away 
some  of  the  heat.  If  the  air  were  immovable,  very  little 
would  be  thus  carried  away  through  so  poor  a  conductor; 
but  as  soon  as  a  layer  of  air  near  the  ground  is  warmed, 
it  rises,  cooler  air  forcing  it  up  and  taking  its  place ;  and 
hence  much  heat  is  thus  removed  and  distributed  from 
place  to  place. 

The  warming  of  the  land  is  more  effective  in  some  situations  than 
in  others.  In  the  case  of  black  and  light-colored  rocks,  we  have 
instances  of  two  extremes.  The  warmth  of  the  land  also  depends 
upon  its  outline.  The  hilltop,  being  more  exposed  to  the  wind,  and 
being  able  to  radiate  heat  through  less  air  than  that  which  covers  the 
valley  bottom,  is  cooler  than  the  valley.  The  increased  warmth  of  the 
valley  also  partly  depends  upon  the  fact  that  the  sides  rejlect  heat  into 
the  valley,  and  check  radiation  from  it,  while  the  hilltop  is  open  .to 
the  sky,  and  can  radiate  heat  in  all  directions.  A  valley  facing  toward 
the  south,  and  hence  exposed  to  the  direct  rays  of  the  sun,  becomes 
warmer  than  one  facing  toward  the  east  or  west,  and  hence  in  shadow 
during  part  of  the  day. 

Warming  of  the  Ocean.  —  For  various  reasons,  water 
increases  in  temperature  much  less  rapidly  than  land.  In 
the  first  place,  heat  rays  are  more  readily  reflected  from 
the  smooth  and  often  glassy  water  surface.  The  heat  that 
is  absorbed,  and  causes  a  rise  in  temperature,  also  expands 
the  water.  Since  it  is  a  mobile  liquid,  it  is  then  set  in 
motion,  and  currents  are  caused  as  a  result  of  the  change  in 
density  thus  produced.  So,  while  on  the  land  some  places 
become  warmer  than  others,  on  the  level  ocean  there  is  a 


62  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

uniformity  of  conditions,  the  material  being  all  alike,  and 
so  movable  that  if  one  part  becomes  warmer  than  another, 
the  difference  is  quickly  equalized  by  means  of  a  current. 

There  is  still  another  reason  why  water  temperature  is 
less  easily  raised  than  that  of  the  land.  It  takes  much 
more  heat  to  raise  the  temperature  of  a  certain  bulk  of 
water  one  degree,  than  it  does  the  same  amount  of  earth 
or  rock.  Also,  water  can  be  evaporated,  and  in  doing 
this,  much  heat  energy  is  used;  but  this  heat  is  not  made 
apparent  by  an  increase  of  temperature,  and  so  is  called 
latent  heat  or  heat  of  evaporation.  The  vapor  thus  pro- 
duced rises  into  the  air  and  passes  away  in  the  winds,  so 
that  when  it  finally  changes  back  to  liquid  water,  the 
latent  heat,  which  then  becomes  apparent,  may  appear  at 
some  very  distant  point  and  warm  the  air,  instead  of  the 
ocean  whence  it  came.  Hence  much  of  the  solar  heat  that 
enters  the  water  is  borne  away  in  vapor. 

For  these  reasons  water  warms  very  slowly,  and  even 
in  midsummer  the  sea  breeze  that  blows  upon  the  land 
from  the  ocean,  is  a  cool,  refreshing  breath  of  air.  At 
night,  and  in  winter,  the  water  cools  to  a  less  degree  than 
the  land,  because  nidiation  from  its  surface  is  less  rapid. 
Therefore  in  day  and  summer  the  ocean  is  relatively  cool, 
and  in  winter,  and  during  the  night,  it  is  warmer  than  the 
land.  Hence  the  climate  of  the  ocean  is  equable,  and  one 
of  slight  change;  and  the  influence  of  this  uniformity 
is  felt  upon  the  land  which  borders  the  sea.  Because  of 
the  same  peculiarity,  even  lakes  of  small  size  influence 
the  local  climate  perceptibly.  "/ 

Temperature  of  Highlands.  —  After  a  storm  in  the  moun- 
tains, or  even  in  a  hilly  country,  one  may  often  see  that 
snow  has  fallen  on  the  hill  or  mountain  tops,  while  rain 


SUN'S  HEAT 


63 


fell  in  the  valleys.  If  one  should  carry  a  thermometer 
on  a  journey  to  a  mountain  top,  he  would  find  the  tem- 
perature steadily  descending;  and  the  same  condition  is 
noticed  by  balloonists  who  ascend  high  into  the  air.  By 
such  observations,  it  is  proved  that  the  temperature  of  the 


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Fig.  18. 

Ideal  section  of  atmosphere  in  temperate  and  equatorial  regions,  showing 
temperature  at  the  surface  and  at  various  elevations. 

air  decreases  with  elevation  above  the  sea.  The  rate 
varies  somewhat,  but  on  the  ayerage  there  is  a  decrease 
of  about  1°  for  every  300  feet  of  elevation.  Even  in  the 
hottest  parts  of  the  earth,  mountains  may  rise  high  enough 
to  reach  the  region  of  eternal  snow. 

This  cold  is  partly  inherent  in  the  thin  upper  layers, 
and  it  is  partly  due  to  distance  from  the  warm  earthy 
which  causes  the  temperature  of  the  air  near  the  ground 
to  rise.  The  low  temperatures  of  mountain  tops  are  in- 
tensified  hy  radiation^  for  the  direct  rays  of  the  sun  reach 
the  mountains,  just  as  they  do  the  lowlands,  though  after 
having  passed  through  a  thinner  layer  of  atmosphere. 
This  heat  is  rapidly  radiated,  because  radiation  proceeds 
with  much  greater  ease  through  the  thin  layer  of  rarefied 


64  FIBST  BOOK  OF  PHYSICAL   GEOGBAPHT 

upper  atmosphere,  than  it  does  through  the  denser,  dust- 
filled  air  near  the  sea  level.  Hence  the  nights  and  win- 
ters on  mountain  tops  are  intensely  cold,  partly  because 
of  the  cold  air  which  surrounds  them,  and  partly  be- 
cause of  the  easy  radiation. 

Effect  of  Heat  on  the  Air.  —  On  a  very  clear  day,  the  noon- 
day sun  sends  its  rays  through  the  air  with  little  interrup- 
tion, and  the  larger  share  of  those  that  enter,  reach  the 
ground.  Being  clear,  radiation  and  reflection  from  the 
surface  are  also  easy,  and  much  heat  goes  directly  back, 
while  at  night,  if  the  same  condition  of  the  atmosphere 
exists,  radiation  continues  to  proceed  rapidly.  Thus  on 
a  midsummer  day,  with  the  sun  shining  brightly,  the  air 
may  be  refreshing  both  by  day  and  night.  The  heat  is 
not  imprisoned  nor  entrapped. 

If  the  sky  is  overcast  with  clouds,  the  heat  rays  pass  with 
difficulty,  and  because  so  little  heat  reaches  the  ground 
the  day  is  liable  to  be  cool.  After  such  a  day,  although 
little  sunlight  has  passed  to  the  earth,  the  night  time 
is  usually  not  cold,  because  the  cloud  covering,  which 
prevented  rays  from  entering,  also  acts  as  a  blanket,  and 
prevents  the  escape  by  radiation,  of  the  heat  which  had 
previously  come  to  the  earth.  At  such  times  the  tem- 
perature of  day  and  night  may  remain  about  the  same, 
because  little  heat  comes  and  little  leaves. 

If  the  day  is  hazy,  or  if  the  air  contains  much  vapor, 
and  is  "muggy,"  this,  to  a  certain  extent,  checks  the  pas- 
sage of  rays  from  the  sun;  for  the  foreign  particles  absorb 
some  of  the  heat  as  it  passes ;  but  much  heat  still  reaches 
the  earth,  and  the  impure  air  acts  also  as  a  barrier  to  its 
outward  passage  by  radiation.  So  on  such  days,  the  ground 
and  air  are  warm,  and  the  day  oppressive.     Since  mdiation 


sun's  heat  65 

is  partly  checked,  even  the  night  time  may  offer  no  relief 
from  the  oppressive  heat;  and  man  and  beast  suffer,  until 
finally  relief  comes  when  the  air  is  again  clear,  and  some 
of  the  excessive  heat  can  be  radiated  into  space,    i^ 

An  atmosphere  composed  of  pure  nitrogen  and  oxygen, 
would  offer  little  resistance  to  the  passage  of  heat  rays, 
because  these  gases  are  very  diathermanous ;  and  in  this 
case  the  air  would  be  very  slightly  warmed  by  the  passage 
of  sunlight  through  it.  But  water  vapor  and  dust  parti- 
cles exist  in  the  air,  and  these  intercept  some  of  the  heat 
and  thus  become  warmed.  This  heat  is  to  some  extent 
imparted  to  the  neighboring  air  by  conduction.  These 
foreign  substances  intercept  both  the  direct  rays  from  the 
sun  and  those  radiated  from  the  earth,  so  that  there  is  a 
double  cause  for  warming. 

However,  the  air  receives  its  warmth  mainly  in  an 
indirect  way.  On  a  hot  summer  day,  a  thermometer  placed 
a  foot  from  the  ground  registers  a  considerably  higher 
temperature  than  one  10  feet  above  it;  for  the  lower  la3^er 
is  warmed  by  contact  with  the  heated  earth.  So  by  con- 
duction the  air  temperature  is  raised,  and  then  by  convec- 
tion the  warm  lower  layers  rise,  and  the  heat  from  the 
ground  is  distributed,  just  as  the  air  of  a  room  is  warmed 
by  a  stove,  or  by  a  steam  radiator. 

There  are  many  differences  in  warmth  of  the  air  from 
place  to  place,  from  hill  to  valley,  from  land  to  ocean, 
from  one  kind  of  ground  to  another,  and  from  one  latitude 
to  another.  By  means  of  these  differences  in  temperature, 
the  elastic  air  is  set  into  motion,  and  directly  or  indirectly 
winds,  clouds,  and  storms  are  caused.  The  air  is  there- 
fore a  carrier  of  heat,  and  it  is  always  at  work  equallizing 
the  differences  in  temperature,  and  hence  in  density. 

3- 


66 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


If  in  their  passage  through  the  atmosphere  the  heat  rays 
are  checked  by  dust  particles  when  the  sun  is  nearly  ver- 
tical, and  the  thickness  of  the  air  is  least,  we  can  easily 
understand  that  at  night,  or  early  in  the  morning,  when  the 
rays  pass  through  so  much  more  air  (Fig.  21),  their  effect 
is  greatly  decreased.  This  is  one  of  the  reasons  ^  why  the 
afternoon  sun  of  summer  loses  its  power  as  it  sets  toward 
the  west,  and  why  the  morning  sun  does  not  quickly 


100° 
95° 
90° 
86^ 
80° 
75° 
70° 
65° 
60° 
65^ 

NOON 

NOON 

NOON 

NOON 

NOON 

NOON 

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90^ 

85° 

80° 

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Fig.  19. 

Diagram  showing  change  in  temperature  for  six  successive  summer  days,  the 
temperature  rising  in  the  morning  and  early  afternoon,  and  then  descend- 
ing until  the  sun  again  rises. 


warm  the  earth  and  air.  It  is  also  one  of  the  reasons 
why,  even  at  noonday,  the  winter  sun  does  not  generally 
warm  the  land;  for  then  the  sun  is  low  in  the  southern 
heavens,  and  the  amount  of  air  through  which  the  rays 
must  pass  is  similar  to  that  of  the  afternoon  sun. 

Effect  of  Rotation.  —  By  the  earth's  rotation,  in  most 
parts  of  the  earth,  the  sun  each  day  mounts  higher  and 
higher  in  the  heavens,  first  warming  the  earth  feebly,  as 
its  rays  traverse  the  great  thickness  of  lower  dense  arid 
dusty  air,  then  increasing  in  intensity  until  afternoon, 
and  again  losing  intensity  as  the  horizon  is  neared. 

^  Another  is  the  different  angle  at  which  the  rays  reach  the  surface. 


sun's  heat  67 

There  is  another  important  action  of  rotation  which  affects  the 
moving  currents  of  air  and  water.  In  the  northern  hemisphere  it 
causes  them  to  turn  to  the  right,  in  the  southern  toward  the  left;  or 
in  other  words,  if  they  are  moving  toward  the  Equator,  they  are 
turned  toward  the  west ;  if  from  the  Equator,  toward  the  east.  The 
deflective  influence  is  greatest  near  the  poles  and  least  near  the  Equa- 
tor. It  varies  also  with  the  velocity  of  the  moving  current,  being 
greatest  with  those  that  move  most  rapidly.  We  shall  have  occasion 
to  call  attention  in  several  places  to  the  effect  of  this  right-hand  and 
left-hand  deflection  of  air  and  water  currents.^ 

Effect  of  Revolution.  —  Every  part  of  the  earth  has  its 
temperature  influenced  by  revolution,  for  this  causes  sea- 
sons (p.  1.7).  At  all  times  the  belt  near  the  Equator 
receives  more  heat  than  that  near  the  poles,  for  in  the 
latter  the  rays  never  reach  the  earth  from  overhead,  while 
in  the  former  the  sun  is  never  far  from  vertical  at  midday. 
The  angle  at  which  the  sunlight  reaches  the  earth  during 
the  polar  summer,  corresponds  in  a  measure  to  that  of  the 
light  which  comes  to  us  late  in  the  afternoon;  but  in 
winter  no  sunlight  reaches  the  polar  regions. 

Because  of  the  difference  in  the  amount  of  heat  re- 
ceived, the  earth  is  divided  into  five  great  zones :  (1)  the 
Tropical^  Equatorial^  or  Torrid ;  (2)  the  North  Temperate^ 
between  the  Arctic  circle  and  the  Tropic  of  Cancer; 
(3)  the  South  Temperate^  between  the  Antarctic  circle 
and  the  Tropic  of  Capricorn;  (4)  the  Arctic  or  North 
Polar  zone^  within  the  Arctic  circle ;  and  (5)  the  Antarc- 
tic or  South  Polar  zone. 

So  the  solar  rays  vary  in  effect  from  one  latitude  to 
another,  decreasing  toward  the  poles;  but  this  variation 
is  partly  checked  by  the  revolution,  which  in  all  other 

1  No  attempt  is  made  to  explain  this  phenomenon  in  this  book,  for  the 
explanation  is  difficult  to  give  without  the  use  of  mathematics. 


68  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

zones  than  the  Tropical,  produces  seasons  of  alternate 
warmth  and  cold.  Ordinarily  the  sun  rises  and  sets ;  but 
within  the  Arctic  and  Antarctic  circles  the  revolution  of 
the  earth,  with  its  axis  inclined,  destroys  the  difference  be- 
tween day  and  night  during  a  part  of  the  year.  Even  out- 
side of  these  zones,  the  relative  length  of  day  and  night 
is  caused  to  constantly  vary,  even  as  far  as  the  Equator; 
and  in  our  latitude  we  have  long  nights  in  one  season  and 
short  nights  in  the  opposite.  In  the  United  States  the 
noonday  sun  of  summer  shines  from  a  point  near  the 
zenith.  Six  months  later  it  is  far  down  in  the  southern 
heavens,  and  day  by  day  these  conditions  change. 

From  this  statement  it  will  be  seen  that  the  question 
of  temperature  distribution  over  the  earth  is  a  complex 
one,  and  that  there  must  be  many  variations  from  day  to 
night,  from  season  to  season,  from  one  latitude  to  another, 
from  mountain  to  plain,  and  from  land  to  sea.  But  if  we 
understand  the  principles  which  have  been  dwelt  upon  in 
this  chapter,  we  may  study  and  understand  the  variations 
in  the  temperature  of  the  earth's  surface. 

Temperature  Measurement.  —  Various  instruments  are  in  use  for 
determining  the  temperature  of  the  air;  but  the  ordinary  mercMnaZ 
thermometer  is  the  most  common  and  useful.  A  glass  tabe,  sealed 
at  both  ends,  and  terminating  in  a  bulb  at  the  base,  contains  mercury 
with  a  partial  vacuum  above.  The  principle  of  its  action  is  tliat 
liquids  expand  with  warmth  and  contract  with  cold.  Hence  an  in- 
crease in  air  temperature  causes  the  mercury  to  rise  in  the  tube,  and 
the  lowering  of  the  temperature  makes  it  necessary  for  the  mercury  to 
sink.  The  liquid  thread  which  rises  and  falls,  is  forced  up  and  down 
by  the  expansion  and  contraction  of  mercury  in  a  cistern  or  bulb. 

Either  the  glass  tube,  or  the  standard  upon  which  it  is  mounted,  is 
graduated  into  a  scale,  the  one  in  ordinary  use  in  English-speaking 
countries  being  the  Fahrenheit,  in  which  the  freezing  point  is  32°,  and 
the  boiling  point  212°.     Certain   temperatures   are   carefully  deter- 


SUN'S  BEAT 


69 


mine'l,  and  marked  on  the  tube,  and  then  the  remainder  of  the 
scale  is  graduated.  On  the  European  continent,  the  Centigrade  scale  is 
comnronly  used,  this  having  0°  for  freezing  point,  and  100°  for  boiling. 
It  is  a  more  simple  scale,  and  is  coming  into  use  in  this  country. ^ 
Since  the  degrees  recorded  by  warm  conditions  are  high,  we  speak  of 
high  temperatures  as  synonymous  with  warmth,  and  low  temperatures 
with  cold.  Other  liquids  can  be  used,  and  alcohol  is  commonly  em- 
ployed where  very  low  temperatures  are  encountered,  for  then  mer- 
cury freezes  and  ceases 
to  act,  while  alcohol 
does  not. 

A  thermometer  is 
now  made  of  metal,  with 
a  clock  face  over  which 
a  hand  moves.  The 
operation  of  this  de- 
pends upon  the  expan- 
sion and  contraction  of 
metal  strips,  and  this 
change  is  conveyed  to 
the  hand,  which  moves 

ove-  a  graduated  dial.  Such  metal  thermometers  maybe  connected 
wit  1  a  pen  point,  which  presses  against  a  moving  paper  run  by  clock- 
work. These  thermographs  automatically  write  a  continuous  record 
of  the  temperature  changes,  the  time  of  which  may  be  told  because 
the  paper  is  moved  regularly  by  clockwork. 

There  are  thermometers  constructed  to  register  the  highest  tem- 
perature of  the  day  (maximum  thermometers),  and  others  to  register 
the  lowest  (minimum  thermometers).  The  hlack  bulb  thermometer  va^j 
be  used  to  tell  the  intensity  of  the  sun's  rays,  and  also  for  the  purpose  of 
recording  the  amount  of  sunlight.  The  instrument  consists  of  two 
thermometers,  an  ordinary  one,  and  one  with  a  bulb  blackened  by 
paint  or  lampblack.  Both  are  exposed  to  the  sunlight  side  by  side, 
and  since  the  blackened  bulb  absorbs  more  heat  than  that  of  the 
ordinary  thermometer,  its  record  of  temperature  is  higher. 


Fig.  20. 
A  thermograph. 


^  To  convert  the  Centigrade  to  the  Fahrenheit  scale,  multiply  by  1.8* 
and  add  32°. 


CHAPTER  VI 


TEMPERATURE  OP  THE  EARTH' S  SURFACE 


Day  and  Night  Change  :     Daily  Range.  —  When  the  sun 
rises  above  the  horizon  in  the  morning,  the  temperature  of 


TEMPERATURE 


Diagram  to  show  difference  in  amount  of  air  ( TS)  passed  through  by  rays 
reaching  the  earth's  surface  {SS),  nearly  vertically  {BA)  and  obliquely 
{CA). 

both  land  and  air  is  increased,  though  from  the  effect  of 
radiation  during  the  preceding  night,  it  takes  some  time 

for  the  sun's  heat  to  warm 

them.     The   temperature 

continues    to    rise    until 

mid-afternoon,  and  then, 

as   the   sun's    rays   reach 

the    earth    at    a    greater 

angle,  and   after  passing 

through      an      increased 

Fig.  22.  thickness  of  air  (Fig.  21), 

the  effect  is  lessened,  and  radiation  exceeds  the  supply  of 

heat.     Then   the   temperature   descends,   slowly   at  first, 

70 


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y 

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TEMPERATURE  OF  THE  EABTH's  SURFACE         71 


then  more  and  more  rapidly  as  the  sun  sinks  behind 
the  western  horizon  (Fig.  22).  Radiation  proceeds  to 
lower  the  temperature  until  the  sun  again  rises,  and 
therefore  the  coldest 
period  is  that  just  be- 
fore the  sunrise.  By 
this  means  we  have  a 
normal  rise  and  fall 
of  the  temperature 
each  day  that  the  sun 
shines.  But  there  are 
many  causes  which 
modify  this  normal 
change  or  range^  so 
that  it  differs  from 
place  to  place  (Figs. 
23  and  24),  and  even 
in  the  same  place, 
from  time  to  time 
(Figs.  19,  26,  and 
27). 

Change  with  the 
Seasons. —  This  curve 
or  range  is  not  the 
same  in  all  latitudes, 
but  varies  with  the 
position  of  the  sun  in 
the  heavens.  Within 
the  tropics,  at  all  sea- 
sons, the  midday  tem- 
perature is  very  high, 
and    after  sunset    it 


M       3  A.M.         6             9         NO,ON         3              6             9     P.M.| 

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90° 
80° 
70° 
60 
50' 
40 
30 
20° 

to 
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30° 

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Fig.  23. 

Summer  (heavy  line)  and  winter  (dotted  line) 
daily  range  of  temperature  for  several  places. 
(1)  Arctic  day ;  (2)  St.  Vincent,  Minnesota ; 
(3)  Djarling,  India;  (4)  Jacobabad,  India; 
(5)  Key  West,  Florida;  (6)  Galle,  India. 
5  and  6,  near  the  warm  sea. 


72 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


f-loes  not  fall  low  enough  to  produce  cold  nights.  Within 
the  temperate  zones  a  warm  day  may  be  followed  by  a  frosty 
night.  As  the  season  changes,  the  daily  range  in  temper- 
ature varies.  During  the  summer,  the  normal  change  is 
from  hot  midday  to  cool  night ;  but  in  winter  the  change 
is  from  very  cold  niglit  to  cool  or  cold  midday.  In  the 
polar   zones,  the  summer  day  is  Cool,  and  since  the  sun 

does  not  set,  the  night 
temperature  is  but  lit- 
tle lower.  In  the  win- 
ter, the  temperature 
of  day  and  night  may 
be  exactly  the  same ; 
but  between  these  two 
extremes,  when  the 
sun  does  rise  and  set, 
a  cool  midday  is  fol- 
lowed by  a  cold  night. 
Effect  of  Land  and 
Water.  —  There  is  also 
a  change  with  altitude, 
for  even  at  midday  in 
summer,  the  tempera- 
ture of  highlands  and 
mountains  is  not  high, 
and  the  nights  are 
cool,  because  these 
rise  into  the  cool,  thin,  upper  layers  of  air,  where  radia- 
tion is  always  very  rapid.  In  the  winter  similar  change  is 
noticed,  though  the  temperatures  are  lower. 

Between  the  ocean  and  the  inland,  there  is  also  a  de- 
cided difference  in  the  daily  temperature  changes  (Figs. 


Fig.  24. 

Diagram  to  show  influence  of  ocean  on  daily 
temperature  range  in  summer  and  winter. 


TEMPERATURE  OF  THE  EARTH'S  SURFACE         73 


M              6  A.M.         NOON          6  P.M.           M 

loo' 

90 

so' 

70° 

/^^\ 

vA     _X 

Y\ 

^  k^ 

V 

/ 

\ 

lB        / 

23  and  24).  The  ocean  warms  slowly,  and  it  does  not 
cool  rapidly,  while  the  land  warms  and  cools  with  much 
greater  rapidity.  So  the  daily  temperature  range  is  greater 
in  the  interior  of  continents,  than  on  the  ocean,  or  on  land 
that  is  influenced  by  the  neigh- 
boring sea.  Desert  lands,  being 
covered  by  a  blanket  of  cool 
dry  air,  are  greatly  warmed  in 
the  day  and  cooled  at  night, 
because  the  heat  of  the  sun 
passes  easily  through  this  air, 
and  is  radiated  with  equal  ease ; 
but  humid  lands  are  not  subject 
to  so  much  change,  because  they 
are  blanketed  by  a  vapor-laden 
air.  Therefore,  the  daily  tem- 
perature range  of  the  Sahara  is 
considerably  greater  than  that 
of  the  tropical  belt  of  heavy 
rains  in  northern  Africa,  south 
of  the  desert.     In  the  latter  the 

temperature  of  both  day  and  night  is  high,  wliile  in  the 
Sahara  very  hot  days  are  followed  by  relatively  cool 
nights  (Fig.  25).  ^ 

Irregular  Changes.  —  There  are  many  reasons  why  the 
daily  rise  and  fall  of  temperature  may  be  checked.  If  the 
sky  becomes  cloudy  at  midday,  the  sun's  heat  is  partly 
prevented  from  reaching  the  land,  and  the  temperature 
may  not  continue  to  rise,  or  if  it  does,  it  rises  very  slowl3\ 
A  cool  breeze  may  check  the  rise  of  temperature,  so  that 
the  hottest  part  of  the  day  is  reached  before  noon.  Clouds 
at  night,  or  a  warm  breeze,  may  prevent  the  temperature 


Fig.  25. 
Diagram  to  illustrate  greater 
daily  range  in  temperature 
in  a  dry  desert  climate  {B) 
than  in  an  equally  hot,  but 
humid,  tropical  land  {A). 


74 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


from  descending  as  it  should.  A  humid  or  muggy  air 
causes  a  smaller  range  of  temperature  than  a  clear,  dry  air; 
and  when   humid   conditions   prevail,  the  day  and  night 


M         NOON          M         NOON          M         NOON         M         NOON          M         NOON          M        NOON          M         NOON          M  '      NOON         M    1 

90 

85 
80 
76 
70 
65 
60 
55 
50 
»5 

90 
86 
30 
75 
70 
65 
90 
55 
50 
45 

1 

"^ 

/i 

A 

/    ^ 

f 

/ 

\ 

r 

'^ 

r\ 

/ 

\. 

r 

/ 

\ 

/ 

\ 

/ 

v- 

^S 

\ 

-r^ 

. 

J 

\ 

/ 

\ 

^1 

^^ 

/ 

\ 

r 

"V 

/ 

V 

\/ 

v 

/ 

y\/ 

^ 

V 

\ 

/ 

,     / 

\ 

/ 

\ 

/ 

o 

AUG.  20. 1895 

21 

22 

23 

24 

26 

»• 

AUG.  27. 1895      | 

Fig.  26. 

Diagram  to  show  irregularities  of  the  daily  temperature  range.     Compare 

with  Fig.  19. 

temperature  may  change  very  slightly.  Again,  a  storm  or 
a  cold  wave  may  so  interfere  with  the  daily  range,  that  the 
temperature  may  rise  at  night  and  fall  in  the  day.  There- 
fore, in  reality,  only  a  portion  of  all  the  days  exhibit  the 


Fig.  27. 

Diagram  to  .show  regular  daily  range  of  temperature  and  irregularities  due  to 

various  causes.     Notice  April  22d  and  27th. 

normal  rise  and  fall  of  temperature,  because  this  range  is 
checked  or  altered  in  many  cases,  and  as  the  result  of 
various  causes  (Figs.  26  and  27). ^ 

Seasonal  Temperature  Change :  Seasonal  Range.  —  As  in 
the  case  of  day  and  night,  the  temperature  of  the  year  rises 

1  The  teacher  can  use  these  diagrams  to  test  the  ability  of  the  students 
to  detect  illustrations  of  the  points  discussed  in  the  text. 


TEMPERATURE  OF  THE  EARTH'S   SURFACE        75 

and  falls.  The  cold  ground  of  winter  begins  to  warm  as  the 
sun  rises  higher  in  the  spring,  and  the  rays  reach  the  earth 
more  nearly  verticallj^,  after  passing  through  less  air.  As 
this  continues,  the  ground  and  air,  cooled  during  the  pre- 
ceding winter,  become  warmer ;  and  then,  even  after  mid- 
summer (or  June  20),  the  temperature  continues  to  rise, 
although  i/he  sun's  rays  reach  the  earth  less  vertically. 
Therefore,  the  warmest   time  of  year   does  not  coincide 


■  MN. 

n». 

MAR. 

•»(i. 

M»r 

JUNE 

JULY 

*uo. 

»tl>T. 

OCT. 

new. 

occ. 

-    »• 

.0- 



-^ 

''^ 

.^^^^^^ 

20 

o" 

20 
40' 

■^ 

/ 

^ 

^ 

s 

"^ 

/ 

[/ 

N 

\ 

/ 

\ 

s 

\y 

X 

^-^ 

Fig.  28. 
Seasonal  temperature  range  at  several  places.    Singapore,  in  tropical  ocean. 

with  midsummer,  but  occurs  later.  After  that,  the  tem- 
perature decreases  as  the  rays  reach  the  earth  less  verti- 
cally, and  the  days  become  shorter.  We  have  our  coldest 
days  in  January,  because  radiation  is  in  excess  of  the  sup- 
ply of  heat,  and  the  ground  and  air  continue  to  cool  even 
after  the  shortest  day  (December  21). 

Taking  the  average  of  all  the  temperatures  of  all  the 
days  of  the  year,  we  find  a  gradual  increase  from  winter 
to  summer,  followed  by  a  decrease  until  the  coldest  part 
of  winter.  Such  a  curve,  if  made  accurately,  would  be 
somewhat  irregular,  because  there  are  cool  spells  in  sum- 
mer and  warm  periods  in  winter;   but  in  spite  of   these 


76 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


temporary  variations  there  is  a  progressive  change. 
This  average  seasonal  change  or  curve  (Fig.  28)  may 
vary  a  little  from  one  year  to  another,  but  in  any  given 
place  it  is  nearly  the  same  from  year  to  year,  although,  of 
course,  there  is  much  variety  in  the  temperature  conditions 
from  day  to  day.     But  in  various  parts  of  the  earth,  there 

is  a  very  decided  differ- 
ence in  the  nature  of 
this  seasonal  change, 
dependent  upon  lati- 
tude, altitude,  and  re- 
moteness from  the  sea.^ 
Influence  of  Latitude, 
— At  the  Equator  the 
noonday  sun  is  always 
high  in  the  heavens,  and 
the  days  and  nights  are 
nearly  equal  in  length. 
There  is  enough  change 
in  these  respects  to 
-  ,,  ^  .  ,   ,  cause  seasons;  but  since 

Seasonal  temperature  range  in  several  places. 

(1)  St.  Vincent,  Minnesota  ;  (2)  New  York  the    Summer    and   win- 

state;  (3)  Yuma   Arizona;  (4)  Key  West.  ^  temperatures      are 

Florida;  (5)  Galle,  India.  i 

both  high,  the  range 
from  season  to  season  is  not  great.  Within  the  temperate 
zones  the  sun  is  high  in  the  heavens  in  summer,  and  the 
days  are  long,  while  in  winter  the  sun  is  near  the  horizon 
and  the  nights  are  long.  Hence  the  winter  is  cold  and 
the  summer  warm  or  even  hot. 


JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT 

rsHT 

NOV. 

DEC. 

J^ 

3 

c 
90 

c 
80 

70 

60 

50 

40 

30 

20 

id 

0 

10 

5^ 

5»»* 

^ 

4^ 

i^ 

,__- - 

^ • 

r 

2,. 

-.^^ 

5 

^ 

^4 

X 

r 

f' 

'/ 

"H"" 

N 

^^v 

\ 

\, 

S^ 

y 

v 

\ 

\^ 

<. 

•''  / 

/ 

s 

\ 

*\^ 

2 

.''' 

/ 

\ 

\j 

/ 

/ 

\ 

/ 

\ 

[ 

/ 

\ 

\L 

/ 

Fig.  29. 


1  It  will  furnish  good  practice  to  have  the  students  look  up  the  loca- 
tion, elevation,  etc.,  of  these  places;  and  by  this  they  will  better  appre- 
ciate the  meaning  of  the  differences. 


TEMPERATURE  OF  THE  EARTH'S   SURFACE         77 

Near  the  poles,  within  the  frigid  zones,  the  sun  rises 
high  enough  in  the  heavens  to  raise  the  temperature  con- 
siderably in  summer,  for  then  the  sun  remains  above  the 
horizon  for  weeks,  or  even  months ;  but  during  the  winter 
it  stays  below  the  horizon,  and  then  radiation  cools  the 
land  and  causes  very  low  temperatures. 

Influence  of  Altitude.  —  Altitude  is  almost  as  important 
in  determining  the  seasonal  temperature  of  a  place,  as  lati- 
tude. Even  near  the  Equator  there  may  be  a  frigid 
climate,  with  perpetual  snow  on  the  highest  mountain 
tops ;  and  in  the  temperate  zones  many  of  the  mountains 
reach  the  height  where  snow  can  remain  all  the  year 
round.  In  such  places  the  summer  temperatures  are 
never  high,  while  the  winter  is  -cold.  Therefore  in  the 
same  latitude  the  seasonal  range  may  vary  greatly  with 
elevation  above  the  sea.  In  ascending  mountains  the 
average  temperature  descends,  and  this  change  is  suffi- 
cient to  influence  the  growth  of  plant  life,  so  that  forests 
change  in  nature  and  then  disappear  (Fig.  85),  just  as  they 
do  on  the  way  from  temperate  to  polar  latitudes. 

Influence  of  Land  and  Water.  —  Practically  the  same 
effect  is  produced  by  land  and  water  upon  seasonal  range, 
as  upon  the  daily  change  in  temperature  (Figs.  28,  29,  and 
30).  Even  in  the  temperate  latitudes,  where  there  is  so 
much  difference  between  summer  and  winter,  the  water  of 
the  ocean  does  not  become  higlily  heated  in  summer,  nor 
very  much  colder  in  winter.  This  is  partly  because  the 
water  does  not  Avarm  or  cool  quickly,  and  partly  because 
it  is  in  constant  motion.  Currents  of  cold  water  from  the 
Arctic,  and  of  warm  water  from  the  Tropics,  are  con- 
stantly flowing  in  the  north  Atlantic,  and  influencing  the 
temperature   of   the   air.      Therefore    the   seasonal   range 


78 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


over  the  ocean  is  never  so  great  as  it  is  on  the  land.  One 
may  feel  this  moderating  influence  of  the  water  even  on 
the  shores  of  a  lake,  and  the  influence  of  the  ocean  itself 
is  felt  to  a  considerable  distance  from  the  shore. 


JAN. 

«B. 

MAR. 

APRIL 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

NOV. 

DEC. 

co° 

50° 
40° 
3C° 
20° 

/ 

^^^ 

^'^ 

-=s 

V 

/ 

\ 

l>- 

/ 

' 

\ 

^ 

'v 

^^•^, 

^ 

/ 

-^ 

^ 

^^: 

i^; 

p 

\.i 

N 

\,y 

#' 

Fig.  30. 

Influence  of  water  on  seasonal  range  of  temperature.  Summer  temperature 
at  Nantucket  low,  and  winter  temperature  high  — the  reverse  at  North- 
ampton.    Compare  with  Fig.  24. 

Climatic  Zones.  — Because  of  these  peculiarities  the 
earth  may  be  divided  into  five  great  zones  of  different 
temperature  conditions :  (1)  the  warm  Tropical  belt,  in 
which  the  temperature  is  always  high,  and  the  range  mod- 
erate ;  (2)  the  Temperate  zones  (one  north  and  one  south 
of  the  Equator),  where  the  temperature  is  high  in  summer 
and  low  in  winter,  and  the  range  great,  while  the  average 
temperature  is  moderate ;  and  (3)  the  Frigid  zones  (one 
about  each  pole),  where  the  temperatures  are  always  low 
and  the  range  considerable. 

Each  of  these  zones  must  be  further  subdivided.  In 
each  there  are  insular  or  oceanic  climates,  differing  from 


Facing  page  79. 


H.D.Senott,  K.  r. 


TEMPERATURE  OF  THE  EARTH'S  SURFACE         79 

the  inland  or  continental;  and  there  are  highland  and 
mountain  climates,  differing  from  those  of  the  lowlands. 
Hence  there  is  no  single  feature  of  the  earth,  which  by 
itself  will  determine  the  temperature  conditions.  Lati- 
tude produces  the  most  decided  influence,  and  as  we  go 
from  Equator  to  pole  Ave  pass  through  regions  in  which 
the  average  temperature  is  decreasing.  Still  places  on  the 
same  latitude  do  not  necessarily  have  the  same  climate. 

This  can  be  very  well  illustrated  by  following  a  parallel 
of  latitude,  such  as  the  fortieth  parallel,  which  passes  near 
Philadelphia.  Crossing  the  Atlantic,  where  the  tempera- 
ture is  moderate,  it  enters  Portugal,  where  the  climate  is 
very  w^arm,  then  across  the  plateau  of  Spain,  a  warm,  dry, 
semi-arid  country,  and  emerges  upon  the  warm  Mediterra- 
nean, crossing  the  southern  part  of  Italy,  below  Naples, 
the  place  to  which  people  go  for  the  purpose  of  a  moderate 
winter  climate.  It  then  crosses  Greece  and  Asia  Minor, 
after  which  it  enters  the  dry,  cold  plateau  region  of  central 
Asia,  crossing  China,  near  Pekin,  and  northern  Japan.  The 
parallel  passes  across  the  warm  Pacific,  enters  the  United 
States  north  of  San  Francisco,  where  the  climate  is  very 
equable,  and  ascends  the  mountains  and  plateaus  of  the 
west,  now  passing  over  a  cold  mountain  top,  and  again 
descending  to  a  warm,  dry  plateau  enclosed  between  the 
ranges.  It  then  passes  between  Kansas  and  Nebraska  and 
then  into  Missouri,  central  Illinois,  Indiana,  Ohio,  and 
southern  Pennsylvania.  There  is  every  gradation  from 
the  warm,  almost  tro]3ical  climate  of  the  Mediterranean, 
to  the  frigid  climates  of  the  high  plateaus  and  mountains. 

Isothermal  Lines.  —  An  isothermal  line  is  one  which 
extends  through  places  having  the  same  average  tempera- 
ture.    That  is  to  say,  all  the  temperatures  observed  during 


80  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

a  month  (for  instance)  are  averaged  to  furnish  the  mean 
temperature  for  that  month,  and  the  places  having  the 
same  average  are  connected  by  a  line  which  we  call  an 
isotherm.  In  the  same  way  the  average  temperatures  of  the 
year  give  a  basis  for  the  construction  of  isotherms  for 
the  year.  A  map  in  which  the  lines  of  equal  temperature 
are  drawn  is  called  an  isothermal  chart.  Since  the  tempera- 
ture varies  from  one  place  to  another  on  the  same  latitude, 
the  lines  of  equal  temperature,  or  isotherms,  do  not  pass 
around  the  earth  parallel  to  the  degrees  of  latitude,  but 
in  a  very  irregular  manner. 

Examining  the  charts  which  illustrate  this  chapter,  we 
may  see  how  the  average  temperatui'es  of  the  world  are 
distributed.  The  warmest  belts  are  near  the  Equator,  and 
the  coldest  near  the  poles ;  but  the  hottest  part  of  the 
earth  is  not  exactly  at  the  Equator,  but  north  of  it.  This 
zone  of  greatest  heat  may  be  called  the  Heat  Equator^  and 
the  reason  why  this  is  north  of  the  geographic  Equator,  is 
that  there  is  more  land  in  the  northern  hemisphere  than  in 
the  southern.  This  becomes  warmer  than  the  water,  and 
hence  the  temperatures  in  the  northern  hemisphere  are 
higher,  on  the  average,  than  those  in  the  southern.  Of 
course,  we  know  very  little  about  the  climate  in  the  cold- 
est parts  of  the  earth,  and  nothing  whatever  about  the 
conditions  at  the  poles  ;  and  so  it  is  impossible  to  say  just 
where  the  very  coldest  places  are,  though  the  coldest 
known  place,  or  the  cold  jpole  of  the  earth,  is  in  northern 
Siberia,  near  the  Arctic  circle. 

There  is  an  average  decrease  in  temperature  from  the 
warm  equatorial  region  toward  the  poles,  and  therefore 
the  isotherms  extend  around  the  earth,  following  the  direc- 
tion of  the  parallels  of  latitude  in  a  general  way ;   but 


Facing  page  80 


M.D.Se'vMM.X.T. 


140 loO IJO  [y^L-...  140        liO         IW         80 


rating  page  SI. 


M.V.St...t;f.r. 


TEMPEUATURE  OF  THE  EAttTH^S  SURFACE        SI 

they  do  so  very  irregularly,  and  this  is  particularly  true 
in  the  northern  hemisphere,  where  there  is  so  much  land, 
varying  from  plain  to  mountain,  and  separated  by  water 
bodies.  In  the  southern  hemisphere,  where  there  is  less 
land,  the  isotherms  extend  over  the  water  surface  in  a 
direction  nearly  parallel  to  the  lines  of  latitude.  It  will 
be  instructive  to  follow  one  of  these  isotherms  for  each 
hemisphere ;  and  for  this  purpose  we  may  select  the  iso- 
therm of  50°  (the  one  which  passes  through  all  points 
whose  average  temperature  for  the  year  is  50°),  which 
is  the  one  passing  through  Boston,  Mass. 

Crossing  Massachusetts  Bay  to  tlie  tip  end  of  Cape  Cod 
(between  42°-43°),  this  isotherm  extends  northeastward 
fully  10°,  crossing  the  middle  of  Ireland  and  England. 
It  then  descends  to  the  Black  Sea,  crossing  the  northern 
end  of  the  Caspian,  and  passing  through  central  Asia  near 
the  parallel  of  45°.  It  then  descends  below  the  40th  par- 
allel, crossing  Corea  and  northern  Japan  at  about  40°,  and 
entering  the  Pacific.  Then  passing  northeastward,  the  iso- 
therm enters  this  country  near  the  mouth  of  the  Colum- 
bia and  passes  into  Canada,  then  again  descends  into  the 
United  States,  passing  along  the  southern  portion  of  the 
Great  Lakes. 

In  the  southern  hemisphere  this  same  isotherm  passes 
around  the  earth  nearly  on  the  40th  parallel,  generally  being 
south  of  it,  though  sometimes  passing  to  the  northern  side. 
Therefore  in  the  northern  hemisphere  this  isotherm  ranges 
through  about  15°  of  latitude,  and  in  the  southern  hemi- 
sphere through  not  more  than  8°.  Some  of  the  isotherms 
show  even  a  greater  difference  than  this. 

Comparing  the  January  and  July  isothermal  charts,  we 
may  see  how  much  difference  there  is  between  the  tern- 


82  FIRST  BOOK  OF  PHYSICAL   GFOGBAPHY 

perature  of  the  coldest  and  the  warmest  months.  In  the 
north  Atlantic  the  isotherm  of  40°  for  January  reaches 
above  the  60th  pai:allel,  while  in  China  it  descends  to  the 
30th,  ranging  through  more  than  30°  of  latitude  in  the 
two  regions.  During  July  this  isotherm  is  entirely  within 
the  Arctic  circle,  and  in  some  places  nearly  reaches  the 
80th  parallel.  In  the  southern  hemisphere,  during  the 
southern  summer  (January),  the  40°  isotherm  runs  nearly 
parallel  to  and  a  little  north  of  60°  south  latitude,  while 
in  the  southern  winter  it  is  just  north  of  the  50th  paral- 
lel. So  the  climate  of  the  north  temperate  zone  is  shown 
to  be  more  extreme  than  that  of  the  oceanic  southern 
hemisphere. 

The  study  of  these  charts  reveals  many  other  features. 
Where  they  flow,  warm  ocean  currents  raise  the  tempera- 
ture of  the  sea  and  bend  the  isotherms  toward  the  poles ; 
a  cold  current,  coming  from  the  north,  cools  the  air  and 
bends  the  isotherms  toward  the  Equator.  Each  of  these 
features  is  very  well  shown  in  the  north  Atlantic  on  the 
January  chart.  A  comparison  of  the  charts  shows  how 
cold  the  interior  of  the  continents  becomes  in  winter,  and 
how  warm  they  are  in  summer ;  and  we  find  here  an  excel- 
lent illustration  of  the  difference  between  the  oceanic  and 
inland  climates.  '^ 

The  world  is  so  large  that  such  maps  as  these  can  do  no 
more  than  show  the  most  general  features.  On  those  of 
the  United  States  we  may  find  other  features,  the.  result  of 
minor  causes,  such  as  the  influence  of  highlands  in  dis- 
turbing the  direction  of  the  isotherms.  Thus  on  the 
Pacific  slope  the  isotherms  run  nearly  parallel  to  the 
coast,  because  the  air,  blowing  in  from  the  Pacific,  rises 
against  the   mountain   ranges   and   cools.     These   ranges 


TEMPERATURE  OF  THE  EARTH'S  SURFACE         83 

are  nearly  parallel  to  the  coast,  and  so  the  isotherms 
run  in  this  direction,  each  one  representing  a  greater 
distance  from  the  sea,  and  a  greater  elevation.  If  we 
could  have  even  a  more  detailed  chart  of  a  small  area  of 
country,  such  as  a  state,  we  would  find  many  other  varia- 


FiG.  31. 

Isothermal  chart  of  southern  New  England,  showing  influence  of  high  and 

low  land. 


tions.  In  New  York,  the  Hudson  and  Mohawk  valleys 
are  warmer  than  the  enclosing  highlands,  and  the  shores  of 
the  Great  Lakes  are  more  equable  than  the  country  which 
lies  at  a  distance  from  these  large  water  bodies.  In  New 
England,  the  Connecticut  valley  is  warmer  than  the  high- 


84         nnsT  book  op  piirstcAL  geography 

land  region  on  either  side  (Fig.   31 1),  and   the  same  is 
shown  in  many  other  regions. 

Because  of  the  many  variations  in  heat  effect  described 
in  the  preceding  pages,  there  are  numerous  kinds  of  cli- 
mate,— the  warm  and  equable  ocean,  the  heated  desert,  the 
interior  plain,  with  extremes  of  heat  and  cold,  the  cold 
mountain  tops,  etc.  There  are  also  many  other  indirect 
effects.  The  sea  is  set  in  motion,  winds  are  formed,  vapor 
is  taken  from  the  water,  rains  are  caused,  storms  arise, 
and  the  air  is  constantly  doing  work  of  various  kinds. 

Temperature  Extremes.  —  Owing  to  the  irregular  changes  of 
weather,  which  are  common  in  the  temperate  latitudes,  we  may  have 
a  great  temperature  change  in  a  few  hours.  In  Montana  a  daily 
range  of  50°  is  not  uncommon,  while  at  Key  West,  the  difference 
between  the  record  of  temperature  in  day  and  night  is  generally  from 
5°  to  10°.2  In  Montana  the  range  between  the  highest  temperature 
of  summer  and  the  lowest  of  winter  is  about  145°  (  —  40°  in  winter 
and  105°  in  summer).  At  Key  West  the  range  is  only  about  40° 
(50°  in  winter  and  90°  in  summer).  In  Montana  a  fall  of  100°  has 
been  recorded  in  a  few  days,  while  at  Key  West  there  is  never  a  great 
range.  The  one  place  has  an  insular  climate,  in  the  midst  of  a 
warm  ocean  current,  the  other  is  an  elevated  interior  plain,  far  from 
the  sea,  and  covered  by  relatively  dry  air,  through  which  the  sun's 
heat  falls  readily  to  the  earth  in  summer,  while  in  winter  radiation 
proceeds  with  equal  ease.  Other  parts  of  the  country  show  ranges 
between  these  two  extremes.^ 

1  The  teacher  would  do  well  to  devote  considerable  time  to  the  study 
of  these  charts,  and  to  a  discussion  of  the  many  features  shown,  for 
which  space  cannot  be  given  in  this  book. 

2  In  the  clear,  dry  region  of  Thibet  a  range  of  90°  in  a  single  day  is 
reported,  68°  at  midday  and  —  22°  at  night. 

8  The  highest  air  temperature  ever  recorded  is  127°  in  Algiers,  and  the 
lowest  —  90°  in  central  Siberia,  which  is  the  coldest  known  part  of  the 
earth.  Higher  temperatures  tlian  that  of  Algiers  have  been  recorded 
from  near  the  ground,  for  at  times  the  earth  of  deserts,  particularly  if  its 
color  is  dark,  becomes  so  hot  that  it  is  painful  to  walk  on  the  sand.        > 

V 


CHAPTER  VII 

WINDS 

Air  Pressure.  —  About  us  is  a  mass  of  air  which  presses 
down  upon  every  part  of  the  earth.  At  the  sea  level  the 
pressure  of  the  air  amounts  to  about  15  pounds  on  every 
square  inch  of  surface.  A  man  therefore  bears  a  weight 
of  many  thousands  of  pounds  upon  the  outside  of  his  body, 
but  he  moves  about  without  realizing  this,  because  the 
pressure  is  equal  in  all  directions.^ 

The  pressure  of  the  air  is  not  the  same  in  all  places, 
nor  at  all  times  (Fig.  32).  For  various  reasons  it  changes 
from  day  to  day,  and  is  greater  on  cool,  dry  days  than  in 
stormy  weather.  The  weight  of  the  air  also  varies  with 
altitude.  While  it  amounts  to  about  15  pounds  to  the 
square  inch  near  sea  level,  the  pressure  decreases  as  one 
ascends  a  mountain ;  for  of  course  there  is  less  air  over  a 
mountain  top  than  above  a  neighboring  plain.  It  is  found 
also  that  there  is  a  difference  in  the  air  pressure  even  at 
sea  level  in  different  latitudes.  The  reason  for  this  is 
explained  below. 

Measurement  of  Air  Pressure.  —  If  we  should  fill  with  water  a  glass 
tube  35  feet  long,  having  one  end  sealed,  and  then  invert  it  with  the 

1  One  can  prove  the  existence  of  this  pressure,  if  he  places  his  hand 
upon  the  top  of  a  cylinder  on  an  air  pump,  from  which  the  air  is  then 
exhausted.  The  pressure  is  then  removed  from  the  under  side,  but  the 
weight  of  the  atmospheric  column  above  is  felt,  and  the  invisible  load 
presses  on  the  back  of  the  hand  with  such  force  as  to  be  painful. 

85 


86 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


open  end  in  a  dish  of  water,  there  would  be  a  column  of  the  liquid 
rising  in  the  tube  to  the  height  of  about  30  feet.  This  column  of 
water  represents  the  actual  weight  of  an  air  column  having  the  same 
area  of  cross  section.  Such  a  tube  may  be  called  a  barometer,  and  if 
we  watched  the  top  of  the  water  day  by  day,  we  should  find  that  it 
rose  and  fell,  indicating  a  change  in  air  pressure.  Since  these  changes 
are  in  reality  detected  by  a  similar  instrument,  it  is  common  to  speak 
of  a  change  in  barometer  as  synonymous  with  change  in  pressure  (Fig.  32). 
When  the  air  is  heavy,  the  barometer  rises,  and  we  have  a  high  barom- 
eter; when  it  is  light,  we  have  a  low  barometer. 


BAROMETER 

ITHACA 


^ — . 

^, 

X 

'^^^^^ 

V 

\ 

^ 

\ 

/ 

/ 

N 

^ 

\ 

/ 

^    ^ 

vy 

^w^^ 

1892 

Fig.  32. 

Diagram  showing  change  of  pressure  for  seven  d^ys.    Figures  on  the  side  are 
inches  and  tenths  of  inches :  — 28.9,  29.1,  etc. 


It  is  upon  this  same  principle  that  the  ordinary  pump  is  constructed. 
A  tube  leads  to  the  well,  we  exliaust  the  air  from  the  tube  by  means 
of  the  handle,  and  then  the  air,  pressing  on  the  surface  of  the  well, 
forces  water  into  the  tube.  By  such  an  arrangement  we  cannot  pos- 
sibly draw  water  40  feet  above  the  surface  of  the  well".  A  special  kind 
of  pump  is  necessary  to  raise  water  above  the  height  of  2.5  or  30  feet. 

Such  a  barometer  as  that  described  above  is  too  cumbersome  for 


WINDS 


87 


use,  and  for  it  is  substituted  the  mercurial  barometer.  The  liquid  mer- 
cury is  very  much  heavier  than  water,  and  hence  the  weight  of  tlie 
air  cannot  force  it  so  high  in  a  tube  which  has  a  partial  vacuum  at 
the  top.  While  water  can  be  forced  up  to  a  height  of  about  30  feet, 
mercury  is  made  to  rise  only  about  30  inches.  Any  one  can  make  a 
mercurial  barometer  by  filling  with  mercury  a  glass  tube  33  or  34 
inches  long,  with  one  end  sealed,  and  then  inverting  it  with  the  open 
end  under  the  surface  of  a  small  dish  of  mercury. 

The  ordinary  barometer  is  made  in  exactly  this  way,  although 
there  are  many  changes  in  detail,  in  order  to  increase  its  perfection. 
One  of  these  is  the  mode  of  reading  the  height  of  the  barometer,  or  the 
elevation  of  the  mercury  column 
in  the  tube.  The  barometer  is 
graduated  in  inches,  and  it  is 
possible  to  read  to  tenths  or  even 
hundredths  of  an  inch,  so  that 
very  slight  differences  in  air 
pressure  are  noticed.  In  speak- 
ing of  the  height  of  a  barometer, 
we  say  that  it  reads  29.8,  30.1, 
etc.,  inches.  A  barometer  at  sea 
level  has  a  higher  reading  than 
one  at  an  elevation  above  the 
sea,  and  from  day  to  day  every 
barometer  situated  at  one  place 
is  subject  to  change. 

Even  the  mercurial  barometer 
is  somewhat  cumbersome,  and  is 
especially  unfit  for  transporta- 
tion. When  kept  standing  in 
one  place  it  does  very  well,  but 
it  soon  gets  out  of  order  when 
carried  about.  For  some  pur- 
poses it  is  important  to  transport 

barometers,  especially  when  it  is  desired  to  measure  the  elevation  of 
any  part  of  the  land.  Since  there  is  less  air  above  a  high  land  than 
above  the  sea,  a  barometer  carried  up  a  mountain  side  is  subjected  to 
less  and  less  pressure,  and  the  mercury  correspondingly  sinks  a  certain 


Fig.  33. 

Aneroid  barometer  graduated  in   feet 
(outside)  and  inches  (inside). 


88  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

amount ;  and  hence  for  a  fall  of  a  fraction  of  an  inch,  we  are  able 
to  calculate  the  amount  of  elevation  over  which  we  have  passed.  So 
the  height  of  a  mountain  can  be  determined  by  means  of  the  barometer. 
A  change  in  pressure  of  an  inch  means  an  elevation  of  about  900  feet, 
though  this  varies  somewhat  with  the  amount  of  elevation. 

For  this  class  of  work  a  special  form  of  barometer  has  been  devised, 
called  the  aneroid  (Fig.  33).  Here  instead  of  a  liquid,  the  pressure 
of  the  air  exerts  its  force  on  a  metal  diaphragm  within  a  metal  case. 
The  change  produced  on  the  diaphragm  is  communicated  to  a  hand 
which  moves  over  the  face,  somewhat  as  the  hands  of  a  watch  pass 
over  the  dial.  The  face  beneath  the  hands  is  graduated  to  feet,  so 
that  as  the  index  moves,  a  person  may  read  the  change,  just  as  he 
reads  the  time  on  a  watch.  In  climbing  an  elevation,  the  hand 
moves  in  one  direction,  and  in  descending,  in  the  other.  The  entire 
instrument  is  so  small  that  it  may  be  carried  in"  the  pocket. 

Aneroids  are  also  used  in  recording  a  change  in  pressure.  The 
instrument  is  placed  in  the  proper  position,  and  since  the  pressure 
varies  from  day  to  day,  the  hand  moves  backward  and  forward  ac- 
cording to  these  changes.  On  it  is  fixed  a  pen  which  presses  against 
a  sheet  of  paper  moved  by  clockwork  (as  in  the  thermograph).  The 
pen  therefore  marks  the  changes  in  pressure,  while  the  rate  of  move- 
ment of  the  paper  keeps  record  of  the  time,  so  that  a  continuous 
record  is  kept  both  of  the  time  and  amount  of  change  in  air  pressure. 
Such  a  self-recording  barometer  is  called  a  barograph. 

Change  in  Air  Pressure. — When  air  is  warmed,  the  mole- 
cules are  spread  apart,  and  it  becomes  ligbter.  The  same 
is  true  of  a  liquid.  This  can  be  proved  by  a  simple  exper- 
iment :  place  a  drop  of  colored  ice  water  in  a  glass  of  water 
having  a  temperature  of  40°,  50°,  or  60°,  and  the  colored 
water  will  sink,  because  it  is  heavier  than  the  remainder. 
Also  on  a  cold  winter's  night,  when  the  air  outside  is 
perfectly  still,  if  a  window  in  a  warm  room  is  opened, 
the  heavy  cold  air  from  out  of  doors  pours  into  the  room. 
The  same  principle  is  illustrated  by  a  stove  or  a  lamp. 
The  air  near  this  is  warmed,  and  hence  made  lighter  than 


WINDS  89 

that  which  occupies  the  more  remote  parts  of  the  room. 
The  heavy  air  presses  down  and  forces  the  warm  air  to 
rise,  causing  a  circulation  in  the  room.  By  standing  on 
a  chair  in  a  well-warmed  room,  we  may  see  that  the 
cooler  air  stays  at  the  bottom,  while  the  warm  air  rises  to 
the  top.  Oftentimes  the  upper  layers  are  suffocatingly 
hot,  while  those  near  the  floor  are  only  comfortably  warm.^ 

The  atmosphere  may  be  likened  to  the  air  of  a  room. 
Some  places  are  being  warmed  more  than  others,  and  those 
that  are  most  warmed  are  lighter  than  the  colder  portions. 
For  instance,  the  air  over  the  Mississippi  valley  may  be 
warm,  and  that  over  New  England  cold,  and  we  then  have 
a  high  barometer,  or  heavy  pressure,  in  the  latter  place, 
and  a  low  barometer,  or  low  pressure,  in  the  former  place. 
The  barometer  may  register  a  difference  of  an  inch  in  the 
two  places.  This  difference  in  pressure  gives  rise  to  what 
is  called  the  barometric  gradient^  and  the  air  will  move 
from  the  New  England  region  toward  the  low-pressure 
area  of  the  Mississippi  valley,  causing  winds  from  the 
east.  It  does  this  because  the  atmospheric  gases  are  so 
mobile  that  even  slight  differences  in  pressure  cannot 
long  exist. 

It  is  somewhat  like  pouring  a  glass  of  water  into  a 
basin.  The  water  that  is  poured  in  does  not  stay  in  one 
place,  raising  the  surface  of  that  part  of  the  basin,  but 
flows  about,  and  causes  the  neighboring  layers  to  move, 
with  the  result  that  the  entire  surface  is  raised  a  little, 
and  the  pressure  remains  the  same  in  all  parts  of  the 
basin.  Just  so  in  the  air:  no  sooner  is  a  difference  in 
pressure  introduced  than   movement  begins  to  equalize 

1  This  subject  of  air  circulation  may  well  be  prefaced  by  an  experi- 
mental study,  or  a  consideration  of  ventilation. 


90  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

it,  and  attempt  to  make  the  pressure  everywhere  the  same. 
It  is  upon  this  principle  that  most  of  the  winds  of  the 
globe  depend. 

Planetary  Winds  :  Theoretical  Circulation.  —  The  earth 
is  divided  into  zones:  there  is  cold  about  the  poles  and 
heat  at  the  Equator.  Therefore  we  may  expect  a  great 
planetary  circulation  similar  to  that  in  a  room  containing 
a  stove.  Warmed  most  around  the  equatorial  belt,  the 
air  here  is  expanded  and  made  light,  while  on  either  side, 
north  and  south  of  this,  there  are  belts  with  a  cooler  cli- 

CALM3 
N-*l I^S 

Fia.  34. 

Cross  section  of  atmosphere  showing,  very  diagrammatlcally,  the  general  air 
circulation.    E,  equator;  S  and  iV,  south  and  north  poles. 

mate.  In  the  latter,  since  the  air  is  heavy,  there  must 
be  a  flow  toward  the  warm  region,  in  order  to  equalize 
the  pressure  produced  by  the  difference  in  temperature. 
This  will  force  the  air  near  the  Equator  to  rise,  just  as 
air  ascends  in  a  fireplace,  or  through  the  draft  of  a  stove, 
or  of  a  lamp.  But  if  it  rises,  while  air  from  the  north 
and  south  flows  in  to  become  warmed  and  also  rise,  there 
must  be  some  escape.     Otherwise  so  much  air  will  reach 


WINDS  91 

the  Equator  that  the  increase  in  bulk  will  make  the  press- 
ure as  great  as  that  in  the  colder  temperate  zones. 

The  cause  for  this  imaginary  circulation  of  the  air  is  at 
work  every  day  in  the  year,  and  hence  a  movement  of  a 
very  permanent  nature  must  be  begun.  Again,  we  may 
make  a  comparison  to  a  room  containing  a  stove.  Here 
the  air  is  warmed  near  the  stove,  cold  air  flows  in,  forcing 
up  the  warm  layers,  which  thus  reach  the  ceiling  and  flow 
away  to  the  sides  of  the  room,  while  the  air  that  moved 
in  toward  the  stove  is  warmed  and  also  made  to  rise.  At 
the  ceiling  the  air  is  cooled  by  contact  with  the  cooler 
body;  it  settles,  and  again  flows  toward  the  stove,  so  that 
a  constant  circulation  is  maintained.  Here,  then,  we 
have  four  zones  of  movement:  (1)  the  current  along  the 
floor  toward  the  stove ;  (2)  the  vertical  current  over  and 
near  it;  (3)  an  upper  current  moving  away  from  the  stove, 
above  that  on  the  floor,  and  in  the  opposite  direction;  and, 
finally,  (4)  the  settling  of  the  upper  air  on  the  opposite 
side  of  the  room.  Upon  a  much  larger  scale  this  is  what 
we  may  expect  to  find  on  the  earth,  v-' 

Trade -Wind  Circulation.  —  Near  the  Equator  there  is  a 
belt  of  calms,  which  is  the  place  where  the  air  is  rising. 
This  ascending  air  cools,  some  of  its  vapor  is  condensed, 
and  rain  storms  are  of  daily  occurrence.  It  is  therefore 
not  only  a  belt  of  calm  air,  with  light  and  variable  winds, 
but  also  a  very  rainy  belt,  and  it  is  sometimes  called  the 
doldrums.  Moving  toward  the  doldrums,  on  either  side, 
are  the  trade  winds,  which  flow  over  the  ocean  in  a 
remarkably  permanent  manner,  both  winter  and  summer. 
These  represent  the  cooler,  dense  layers  near  the  earth, 
which  are  moving  in  toward  the  great  terrestrial  stove, 
the  equatorial  belt  of  calms.     Above  these,  and  flowing  in 


92  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

an  opposite  direction,  are  the  anti-trades^  whose  existence 
is  proved  by  the  movement  of  clouds  high  in  the  air,  and 
also  by  actual  observations  on  mountain  tops,  where  peaks 
rise  above  the  trade  winds,  and  enter  the  upper  currents. 
Near  the  tropics,  at  the  place  where  the  trades  begin  to 
be  permanent,  there  are  regions  of  settling  air,  known  as 
the  horse  latitudes  ;  and  thus  the  theoretical  circulation  is 
completed,  and  in  fact  we  find  what  was  predicted  in  theory. 

There  is  a  very  important  difference,  however:  accord- 
ing to  theory,  the  trades  should  blow  southward  in  the 
northern  hemisphere  and  northward  in  the  southern,  mov- 
ing directly  toward  the  Equator,  and  the  anti-trades  should 
blow  in  the  opposite  direction.  In  reality  this  is  not  so; 
the  trade  winds  of  the  northern  hemisphere  blow  toward 
the  southwest^  while  the  southern  trades  move  toward  the 
northwest^  the  northern  anti-trades  toward  the  northeast^ 
and  the  southern  toAvard  the  southeast.  The  reason  for 
this  is  the  influence  of  the  earth's  rotation  (p.  67).  Any 
current  on  the  earth,  whether  of  air  or  water,  is  turned  to 
one  side  as  it  moves,  to  the  right  in  the  northern  hemi- 
sphere and  to  the  left  in  the  southern.  Hence  the  trades 
do  not  approach  the  Equator  along  the  meridians. 

The  belt  of  calms  is  not  stationary,  as  is  the  Equator, 
but  migrates  with  the  seasons  (compare  Plates  8  and  9). 
During  our  summer,  when  the  sun  is  vertical  over  the 
northern  tropic,  the  belt  moves  to  the  northward;  and  in 
winter  it  migrates  to  the  south,  because  the  greatest  effect 
of  ilie  sun's  heat  is  first  north  and  later  south.  Hence  as. 
the  equatorial  belt  of  greatest  heat  migrates,  the  trades 
change  their  position,  in  the  summer  being  further  north 
than  in  the  winter.  This  change  influences  the  climate 
of  various  places  very  perceptibly. 


WINDS 


93 


SOUTHERN  HEMISPHERE 


Prevailing  Westerlies.  —  Since  the  polar  regions  are 
places  of  greater  cold,  we  might  expect  that  in  these  zones 
the  air  pressure  would  be  high;  but  such  is  not  found  to 
be  the  case,  and  we  must  look  for  some  explanation. 
There  certainly  must  be  a  settling  of  dense  air  in  the  high 
temperate  and  polar  lati- 
tudes, because  the  cold  of 
these  regions  makes  the  air 
heavy.  We  know  that  there 
is  a  rising  of  the  air  near 
the  Equator,  but  the  set- 
tling of  this  in  the  region 
of  the  horse  latitudes  does 
not  take  place  so  far  north 
as  this  zone  of  greatest  cold, 
but  rather  in  the  warm  por- 
tions of  the  temperate  zones, 
where  the  trade  winds  begin 
to  blow.  Here,  however, 
only  a  part  of  the  air  settles, 
and  some  of  the  upper  cur- 
rents, which  extend  as  anti- 
trades   from    the     place    of 

equatorial  upflow,  pass  along  toward  the  poles,  constantly 
turning  more  and  more  to  the  east,  under  the  influence  of 
the  earth's  rotation. 

West  winds  therefore  prevail  in  the  upper  air  of  the 
high  latitudes,  and  this  is  also  the  prevailing  direction  of 
the  winds  near  the  ground,  though  there  are  so  many  dis- 
turbing causes  here  that  they  are  not  so  permanent  as  they 
are  high  up  above  the  earth.  Since  both  in  the  higher 
temperate  and  polar  zones  of  the  northern  and  southern 


Fig.  35. 

Ideal  circulation  of  air  near  the  sur- 
face, in  the  southern  hemisphere. 
Trade  :  trade- wind  belt ;  H,  H,  horse 
latitudes  of  uncertain  winds ;  C.W., 
circumpolar  whirl,  or  prevailing 
westerlies. 


94  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

hemispheres,  the  air  movement,  both  aloft  and  near  the 
ground,  prevails  from  the  west,  these  winds  are  called 
the  prevailing  westerlies.  They  complete  the  great  planet- 
ary circulation  of  the  atmosphere,  and  from  them  we 
obtain  an  explanation  of  the  low  pressure  near  the  poles, 
where  at  first  thought  a  high  pressure  would  be  expected. 

The  air  is  moving  toward  the  poles,  where,  because  of 
the  cold,  there  is  greater  density ;  but  as  it  comes  from 
the  broad  temperate  into  the  polar  zones,  it  is  passing 
toward  a  point,  the  pole,  and  is' constantly  coming  toward 
a  narrower  and  narrower  space.  It  would  be  impossible 
for  all  the  air  that  starts  to  reach  the  polar  zone,  and  hence 
some  sinks  and  returns  toward  the  Equator ;  but  much 
keeps  on.  A  deflection  results  from  the  influence  of  rota- 
tion, which  turns  the  currents  to  one  side,  and  a  whirl  is 
begun,  which  is  somewhat  like  that  produced  in  water 
which  is  escaping  from  a  basin.  In  this  case  the  water  is 
flowing  from  a  broad  area  toward  a  narrow  orifice,  and  if 
we  look  at  such  a  whirl,  we  find  that  the  water  surface  is 
lower  in  the  centre  than  on  the  sides,  and  that  around  the 
depression  the  water  is  whirling  in  spiral  currents. 

Conditions  similar  to  this  evidently  prevail  in  the  re- 
gion surrounding  each  pole,  and  the  whirl  of  air,  in  the 
belt  of  prevailing  westerlies,  forms  what  is  known  as 
the  circumpolar  ivhirl.  Because  of  this  spiral  whirling 
movement,  there  is  actually  less  air  near  the  poles,  and 
hence,  notwithstanding  the  fact  that  it  is  colder  and 
denser  than  in  other  parts  of  the  world,  its  pressure  is 
less  than  would  be  expected  if  it  were  not  for  the  whirl. 

The  great  system  of  winds  described  above  is  better 
developed  in  the  southern  than  in  the  northern  hemi- 
spliere,  because  there  is  less  land  in  the  former.     The 


WINDS 


95 


great  expanse  of  water  south  of  the  Equator  allows  the 
air  circulation  to  proceed  with  less  interference  than  in 
the  northern  hemisphere,  where  bodies  of  land  and  water 
alternate  (Plates  8  and  9).  The  heat  of  the  sun  affects 
the  land  much  more  than  the  water,  and  this  difference 
causes  other  winds  from  water  to  land,  or  the  reverse, 


Fig.  36. 

Map  of  Spanish  peninsula,  showing  lines  of  pressure  and  temperature  with 
winds  for  July,  when  the  summer  monsoon  prevails. 


which  mask,  and  at  times  even  destroy  the  great  planetary 
winds  near  the  earth's  surface.  However,  in  both  hemi- 
spheres the  currents  in  the  higher  altitudes  are  remarkably 
permanent,  as  may  be  seen  in  the  United  States  by  watch- 
ing the  passage  of  clouds  high  in  the  air,  which  are 
generally  moving  from  the  west. 

Periodical  Winds :  Monsoons. — The  effect  of  the  sun's 
heat  upon  the  land  is  greater  than  on  the  water,  and  radia- 


96 


FIRST  nooK  OF  PHYSICAL  GlJOGltAPBT 


tion  from  the  land  surface  produces  a  greater  effect  than 
upon  the  Avater.  Hence  if  there  is  a  large  land  area,  in 
summer  it  becomes  warmer  than  the  neighboring  water, 
and  in  winter  cooler.  This  may  be  seen  by  examining 
the  map  of  the  Spanish  peninsula  (Fig.-  36).  There,  in 
summer,  the  air  is  less  dense  over  the  land  than  over  the 
water;  and,  as  in  the  case  of  the  stove,  a  circulation  must 
result.  Therefore  the  denser  air,  which  in  summer  lies 
over  the  Mediterranean  and  Atlantic,  settles  and  forces 


Fig.  37. 
The  summer  (left  hand)  and  winter  (right  hand)  monsoons  of  India. 

the  lighter  air  over  Spain  to  rise ;  but  during  the  winter, 
when  the  land  is  colder  than  the  surrounding  water,  the 
dense  cold  air  over  Spain  settles  and  flows  out  toward  the 
sea,  causing  winds  in  the  opposite  direction.  These  are 
monsoons^  and  they  are  periodical  winds  because  they  blow 
at  certain  definite  periods  of  time. 

The  typical  monsoon  is  found  in  Asia,  but  it  occurs  also 
in  Spain,  Australia,  Texas,  and  elsewhere.  In  Asia,  par- 
ticularly over  India  (Fig.  37),  the  monsoon  winds  are  re- 
markably permanent,  and  are  important  both  in  modifying 


WINDS  97 

the  climate  and  in  navigation.  In  the  warm  part  of 
the  year,  the  summer  monsoon  brings  warm,  damp  air 
from  the  ocean,  and  heavy  rains  result;  but  the  winter 
monsoon,  blowing  from  an  opposite  direction,  bears  cold, 
dry  air  from  the  land.  Year  after  year  these  changes  of 
wind  direction  come  as  regularly  as  the  seasons ;  but  there 
are  of  course  other  winds,  due  to  different  causes,  which 
at  times  bring  air  from  other  directions. 

Land  ayid  Sea  Breezes.  —  People  go  to  the  seashore  to 
escape  the  heat  of  the  summer,  and  they  do  this  because 
the  ocean  does  not  become  so  warm  as  the  land.  This  is 
very  well  illustrated  in  hot  summer  days,  when  the  heat 
of  the  land  is  oppressive,  and  when,  by  going  to  the  coast, 
one  finds  relief  from  the  heat.  Soon,  if  the  day  is  quiet, 
a  gentle  breeze  may  be  seen  ruffling  the  surface  of  the 
sea  (the  same  may  be  seen  on  large  lakes,  like  the  Great 
Lakes),  and  after  awhile  a  cool  draught  comes  from  the 
water.  This  is  the  sea  breeze^  to  which  the  dwellers  by 
the  seashore  look  for  a  relief  from  the  oppressive  heat  of 
the  summer  day.  It  may  reach  a  score  or  more  of  miles 
from  the  coast,  but  its  chief  effect  is  felt  near  the  sea. 

At  such  a  time  the  sea  breeze  may  become  a  strong 
wind,  and  often  in  regions  of  permanent  winds,  such  as 
the  trades,  a  sea  breeze  may  arise  which  succeeds  in 
entirely  changing  the  direction  of  air  movement.  Here, 
as  in  the  monsoon,  the  cause  is  the  heating  of  the  land,  so 
that  the  denser  cold  air  of  the  sea  flows  in  over  the  heated 
land.  Before  it  begins  to  blow,  the  temperature  by  noon 
may  have  mounted  into  the  nineties ;  and  then,  with  the 
coming  of  the  breeze,  the  temperature  may  fall  10°  or  more 
in  less  than  an  hour,  so  tliat  the  time  of  greatest  heat 
does  not  fall  in  the  afternoon  as  usual. 


98 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


At  night,  when  the  land  cools  by  radiation,  the  dense 
air  settles  and  flows  out  toward  the  warm  sea,  causing  a 

land  breeze^  which, 
however,  is  not  so 
pronounced,  nor  so 
frequent  in  its  occur- 
rence, as  the  sea 
breeze.  The  sea  and 
land  breezes  are  also 
felt  along  the  shores 
of  the  greater  lakes, 
though  these  are  per- 
haps more  properly 
called  lake  and  land 
breezes.  Even  near 
some  of  the  smaller 
lakes,  similar  air 
movements  are  noticed,  though  here  only  as  slight 
draughts  of  air.  This  illustrates  how  easy  it  is  for  the 
atmosphere  to  move  when  the  pressure  varies  slightly  by 
differences  in  temperature. 


/-\ 

90 
80 

/ 

''^\Sea  Breeze       \ 

vy^ 

"*"■*"- J 

Fig.  38. 

To  show  effect  of  sea  breeze  upon  the  daily 
temperature  range.  The  normal  range 
shown  by  heavy  line,  the  influence  of  the 
sea  breeze  by  the  dotted  line. 


Mountain  and  Valley  Breezes.  —  Among  mountains,  and  even  in 
hilly  districts,  the  cooling  of  the  air  by  radiation  at  night,  causes  a 
contraction  of  the  lower  layers,  which  becoming  heavier,  flow  down 
hill  toward  the  lower  ground.  Like  water,  the  flowing  air  chooses 
the  valleys  down  which  to  pass,  and  sometimes,  in  a  valley  having 
numerous  branches,  the  breeze  from  the  mountains  and  hills  becomes 
very  strong,  and  even  increases  to  a  gale  before  morning. 

In  the  dajiiime  the  warming  of  the  mountain  sides  sometimes  starts 
a  reverse  movement,  which  gives  rise  to  a  noticeable  but  less  pronounced 
breeze  up  the  mountain  sides.  Those  dwelling  in  hilly  regions  may 
often  feel  the  first-named  wind  during  warm  summer  nights,  but  the 
breeze  moving  up  the  hillside  is  much  less  noticeable. 


WINDS 


99 


Irregular  Winds.  —  There  are  other  winds  of  less  importance,  and 
also  a  great  group  of  winds  having  irregular  directions,  and  associated 
in  cause  with  extensive  storms.  These  are  the  winds  which  are  most 
pronounced  in  the  United  States,  but  their  consideration  must  he, 
postponed  until  we  understand  something  about  storms  (Ch.  VIII). 

Velocity  of  the  Wind.  —  The  velocity  of  the  wind  is 
commonly  stated  in  miles  per  hour,  meaning  the  rate  at 


Oceojv. 


Bluff. 


Rolling  Ground. 


EUZy  Cau/nby. 


Fig.  39. 


Diagram  to  show  two  of  the  several  possible  causes  for  the  wave-like  move- 
ment of  the  air.     Here  the  air  is  disturbed  in  passing  over  hilly  ground. 


which  the  air  travels.  A  wind  with  a  velocity  of  10  miles 
is  one  in  which  the  air  would  move  10  miles  in  an  hour. 
A  slight  breeze  has  a  velocity  of  from  2  to  10  miles,  a 
strong  wind  from  20  to  30  miles,  a  gale  from  40  to  50,  or 
even  very  rarely  60  miles,  while  in  a  tornado  (or  what  the 
newspapers  call  a  "cyclone"),  the  velocity  may  be  100  or 
200  miles  an  hour.  The  rate  at  which  the  wind  travels, 
varies  with  the  difference  in  pressure,  which  the  moving 


100 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


air  is  endeavoring  to  equalize ;  but  this  rate  is  retarded 
near  the  ground  by  the  friction  of  air  with  the  irregular 

earth.  Hence 
on  the  tops  of 
high  towers,  and 
especially  of 
mountains,  the 
wind  blows  with 
much  greater 
force  than  it 
does  on  the 
ground;  and  it 
is  also  stronger 
over  the  smooth 
surface  of  the 
ocean  than  on 
the  rough  land. 


O        >5 


O   '73 
^° 

o  ^  Recent     studies 

'^  ^  have  shown  that 
S  ®  the  wind  is  not  a 
simple  onward 
movement  of  a  reg- 
ular kind,  but  a 
series  of  pulsations, 
somewhat  like  the 
puffs  from  an  en- 
gine. As  the  air 
moves  forward,  it 
—  also  rises  and  falls ; 

and  it  has  been  found  that  even  in  times  of  strong  wind  there  are 
momentary  calms.  We  are  all  familiar  with  this  in  a  larger  way, 
when  the  wind  comes  in  gusts;  but  besides  these,  which  are  well 
known  and  very  noticeable,  there  are  tiny  gusts,  so  slight  that  deli- 
cate instruments  are  needed  to  detect  them  (Fig.  40).     The  vertical 


WINDS 


101 


movement  of  the  air  which  accompanies  this  wave-like  passage  of  the 
winds,  is  believed  to  be  the  motive  power  which  such  birds  as  con- 
dors and  hawks  use  in  their  remarkable  habit  of  soaring  without  the 
movement  of  their  wings.  Perhaps  some  day  men  may  also  make 
use  of  this  principle  in  the  con- 
struction of  some  air  ship. 

Measurement  of  Winds.  — 
Aside  from  certain  delicate  in- 
struments for  special  study  of 
the  wind,  and  from  the  result 
of  wind  studies  in  the  higher 
air  now  being  made  by  means 
of  kites,  there  are  two  instru- 
ments commonly  in  use  for 
studying  the  air  movement. 
One  of  these  is  the  ordinary 
wind  vane,  which  tells  the  direc- 
tion. Every  one  is  familiar 
with  the  construction  of  this 
and  with  its  use.  Sometimes, 
however,  it  is  connected  by 
electricity  in  such  a  way  as  to 
make  an  automatic  record  of 
changes  in  wind  direction.  \^,  /'«\  '      *'>'  \   ]\,}  ,'»  ' 

Another  wind  instrument  is  the  anemometer  (Fig.  41),  which  is' 
used  for  determining  the  rate  at  which  the  air  moves.  This  instru- 
ment consists  of  four  metal  cups  which  are  whirled  about  by  the 
wind,  and  each  revolution  which  they  perform  turns  a  cog  M'heel, 
which  in  turn  moves  others,  causing  a  hand  to  move  over  a  dial  upon 
which  are  figures  representing  miles  and  fractions  of  miles.  A 
certain  number  of  revolutions  of  the  instrument  causes  the  hand  to 
move  over  the  space  marked  on  the  dial  as  one  mile ;  and  therefore 
by  reading  this  dial,  one  can  tell  how  fast  the  wind  blows,  just  as  we 
may  tell  the  time  of  day  by  the  rate  of  movement  of  the  hands  over 
the  dial  of  a  clock.  Oftentimes  the  instrument  is  connected  by 
electric  wire  with  a  self-recording  apparatus,  and  thus  each  revolution 
of  the  anemometer  is  automatically  recorded. 


'FiG.  41. 


CHAPTER   VIII 

STORMS 

Weather  Changes. — In  the  central  and  eastern  states, 
there  is  a  fairly  regular  succession  of  weather  changes, 
though  with  many  minor  variations.  A  cool  (cold  in 
winter)  spell  of  dry  weather  is  folloAved  by  a  rise  in  tem- 
perature which  accompanies  winds  from  southerly  quarters. 
Gradually  the  sky  becomes  overcast,  the  wind  changes 
toward  the  east,  rain  falls,  and  after  awhile  there  is  a 
clearing,  with  lower  temperature,  and  wind  from  the  north 
or  northwest.  During  the  summer  this  change  may  he 
preceded  by.  thunder  storms,  and  in  the  Mississippi  valley 
by  'tornadoes.  In  winter  it  is  often  followed  by  severe 
cold  \7(h;athei",  when, a  blanket  of  cold  air  overspreads  all 
the  eastern  half  of  the  country,  possibly  causing  frosts 
even  in  Florida. 

Every  five  or  six  days  this  cycle  is  passed  through, 
though  perchance  the  rain  may  be  slight,  or  may  not  be 
sufficiently  widespread  to  affect  the  entire  eastern  region. 
Sometimes,  particularly  in  winter,  the  changes  in  tem- 
perature are  rapid  and  severe,  and  at  times  the  force  of 
the  wind  is  great  and  its  effect  destructive,  while  at  other 
times  the  winds  are  only  breezes.  These  weather  changes 
need  to  be  studied  in  considerable  detail,  v/ 

Weather  Maps.  —  The  United  States  government  has  in  its  em- 
ploy a   corps  of  weather   observers,   stationed  at  various   points  in 

102 


STORMS 


103 


the  country,  and  furnished  with  thermometers,  barometers,  and 
other  meteorological  instruments,  to  be  used  in  making  observations 
on  the  changes  in  temperature,  pressure,  wind  direction,  wind  force, 
etc.  These  observations,  made  at  the  same  time  of  day  at  all  stations, 
are  telegraphed  to  headquarters,  and  the  information  thus  obtained 
from  all  parts  of  the  country,  is  placed  upon  a  map,  which  is  called 
the  iveather  map.  These  are  printed  and  widely  distributed,  and 
any  one  sufficiently  interested  may  obtain  them. 


Fig.  42. 

Chart  to  show  weather  conditions  January  7,  1893.  Isobars  (red)  and  red 
shading  show  the  pressure,  heaviest  shading  indicating  highest  pressure. 
Blue  shows  temperature,  heaviest  shade  indicating  lowest  temperature. 
Areas  of  rainfall  dotted.  Arrows  point  in  direction  toward  which  the  wind 
is  blowing. 


Upon  the  weather  map  are  lines  connecting  places  of  equal  tem- 
perature, or  isothermal  lines.  By  these  one  may  tell  what  the  tem- 
perature has  been  in  various  parts  of  the  country.  Isobaric  lines,  or 
lines  connecting  places  having  the  same  air  or  barometric  pressure. 


104 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


are  also  placed  on  the  map.  The  pressure  is  marked  in  inches  and 
tenths  of  an  inch,  thus :  30.4,  29.9,  etc.  Arrows  point  in  the  direction 
toward  which  the  wind  is  blowing,  and  circles  tell  whether  the 
weather  is  cloudy,  rainy,  snowy,  or  clear.  A  stateinent  of  the  weather 
conditions  is  printed  at  the  bottom  of  the  map,  and  a  prediction  for 
the  local  weather  of  the  next  day  is  also  placed  upon  it.  These  maps 
contain  much  valuable  information  about  the  weather,  but  in  some 
respects  they  are  unsatisfactory,  chiefly  because  the  government  does 
not  have  as  many  observers  as  are  really  needed  for  the  work. 


Fig.  43. 

Chafrt  to  show  weather  conditions  January  8,  1893.    Shading,  etc.,  same  as 
-  '  Fig.  42.    Path  of  storm  centre  shown  by  a  series  of  arrows. 

Comparison  of  Weather  Maps.  —  If  we  take  a  series  of 
such  maps  for  successive  days,  we  are  able  to  see  the 
reason  for  the  succession  of  weather  changes  noted  above. 
A  series  of  winter  charts  will  probably  best  illustrate 
the  points,  for  then  the  cycle  of  change  is  most  typical. 


ST0BM8 


105 


In  the  series  selected  for  this  description  (and  each 
winter  will  furnish  many  similar  series),  we  start  with 
one  in  Avhich  the  word  Low  is  placed  in  the  Canadian 
northwest,  north  of  Montana  (Fig.  42).  Around  the 
word  Low,  the  isobaric  lines  are  arranged  concentri- 
cally, the  lowest  pressure  being  within.     Between   the 


Fig.  44. 

Chart  to  show  weather  conditions  January  9,  3893.    Shading,  etc.,  same  as 
Figs.  42  and  43. 


word  Low  and  the  northern  boundary  of  Idaho  the  press- 
ure varies  .4  of  an  inch,  being  29.9  inches  in  the  north, 
and  30.3  in  the  south,  while  further  south  the  pressure 
is  higher.  Toward  the  area  of  Ioav  pressure  the  wind  is 
bloAving  from  various  directions. 

In  the  east,  near  Nova  Scotia,  there  is  another  Low,  and 


106         FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 

here,  also,  the  isobars  are  arranged  around  the  word,  near 
which  is  the  lowest  pressure,  of  29.3  inches.  Toward 
this  also  the  wind  is  blowing  from  all  directions,  and  the 
weather  in  the  neighborhood  of  the  low  pressure  is  rainy. 
The  temperature  here  is  between  10°  and  20°,  while  that 
in  the  other  low-pressure  area  varies  from  10°  to  40°. 

Twenty-four  hours  later,  the  more  eastern  word  Low  has 
disappeared,  and  if  our  map  extended  so  far,  we  would 
find  the  conditions  that  caused  it  out  on  the  Atlantic, 
beyond  Newfoundland  (Fig.  43).  The  western  Low  area 
has  passed  eastward  to  Lake  Superior,  and  with  it  have 
gone  the  conditions  of  cloudy,  rainy  Aveather,  and  for  the 
winter  time,  high  temperature.  In  the  meantime  a  High 
area,  with  clear  Aveather  and  low  temperature,  has  appeared 
in  the  northwest,  near  where  we  first  found  the  low  press- 
ure ;  and  the  next  day  (Fig.  44)  the  Low,  which  is  evi- 
dently a  storm,  has  gone  still  further  east,  while  the  ITigh 
has  also  moved  eastward.  A  day  later  the  Low  is  over 
the  Bay  of  St.  Lawrence,  on  its  way  out  to  sea,  while  the 
clear,  cool  weather  accompanying  the  high-pressure  area, 
has  overspread  New  York  and  possibly  New  England. 
By  this  time  another  Low  will  have  appeared  in  the  north- 
west, and  in  this  way,  day  by  day,  changes  of  similar 
nature  are  recorded  by  the  weather  maps.^ 

1 1  would  urge  upon  teachers  the  advisability  of  obtaining  various 
sets  of  such  maps,  which  each  student  may  study,  so  as  to  become 
thoroughly  familiar  with  the  facts  illustrated,  before  undertaking  the 
study  of  the  nature  of  these  changes.  Also  it  is  of  high  value  to 
have  the  daily  weather  charts  in  the  school.  These  will  undoubtedly 
be  sent  regularly  if  application  is  made  to  the  nearest  Weather  Bureau 
station,  the  location  of  which  can  be  learned  by  addressing  the  chief 
of  the  Weather  Bureau,  United  States  Department  of  Agriculture, 
Washington,  D.C. 


STORMS 


107 


Cyclonic  and  Anticyclonic  Areas :  The  Low-  and  High- 
Pressure  Areas.  —  From  these  observations  it  is  seen,  that 
for  some  reason,  an  area  in  which  the  pressure  is  lower 
than  the  average,  appears  in  the  northwest,  and  progresses 
rapidly  eastward,  passing  over  the  country  in  from  two 
to  four  days.  No  case  is  known  of  such  an  area  starting 
in  the  east  and  going  westward,  though  at  times  they  do 


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Fig.  45. 

Map  showing  paths  followed  by  low-pressure  areas  during  November,  1891. 
Order  of  storms  shown  by  Roman,  and  dates  by  Arabic  numerals.  Figure  1 
beneath  the  line  indicates  morning,  and  2,  evening.  From  these  one  can  tell 
the  direction  and  distance  over  which  each  storm  travelled. 


begin  in  the  southwest,  and  also  in  the  West  Indian 
region.  When  this  is  the  case,  the  low-pressure  area 
moves  toward  the  northeast,  and  then  across  the  Atlantic 
toward  Europe,  which  they  often  reach.  So  Ave  have  as 
one  universal  fact,  a  path  which  ultimately  leads  toward 
the  east  (Fig.  45);  and  what  is  said  of  the  low-pressure 
areas  applies  equally  to  the  high.    The  most  common  path 


108 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


for  these  disturbances  of  the  air  is  eastward  over  the  Great 
Lakes,  through  the  St.  LaAvrence  valley,  over  Newfound- 
land, and  across  the  Atlantic,  toward  northern  Europe. 

In  their  passage  they  sometimes  die  out,  and  on  the 
other  hand  they  at  times  rapidly  develop  renewed  energy. 
In  some   cases  there  is  only  a  slight  difference   in  the 


Fig.  46. 

Weather  conditions  April  20,  1893,  showing  two  high-pressure  areas  and  a 
typical  storm.    Rain  area  shaded.     1.4  inch  difference  in  pressure. 


pressure,  while  at  other  times  the  difference  is  great,  and 
as  these  variations  occur,  there  is  a  change  in  the  velocity 
of  the  wind,  which,  when  the  difference  in  pressure  is 
great,  sometimes  becomes  very  violent.  As  the  low- 
pressure  area  progresses,  the  wind  blows  toward  it  from 
various  directions,  and  when  it  is  typically  developed, 


STORMS 


109 


the  air  moves  from  all  sides  spirally  toward  the  centre  of 
lowest  pressure  (Fig.  46).  Although  the  storm  moves 
eastward,  it  is  not  to  be  inferred  that  the  progress  of  the 
low-pressure  area  across  the  country  is  a  bodily  movement 
of  the  air;  for  if  this  were  so,  there  would  be  extremely 
violent  winds  from  the  west  as  it  passed  along.  What  is 
really  the  case  in  these  disturbances  of  the  air,  is  a  low- 


FiG.  47. 

Map  showing  weather  conditions  November  27,  1896.  From  a  high-pressure 
area  in  the  west,  the  winds  are  blowing  outward.  In  this  high  area  the 
temperature  is  very  low  —  temperature  indicated  by  shading. 

pressure  condition  moving  toward  the  east,  just  as  a  wave 
moves  along  the  water  surface  with  little  real  forward 
movement  of  the  water.  Hence  the  condition  is  this: 
an  area  of  low  pressure  constantly  progresses  eastward 
with  a  wave-like  movement,  and  toward  this  moving  area 
of  low  barometer,  winds  blow  from  all  directions. 


110 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


For  the  high-pressure  area  the  same  holds  true,  except- 
ing that  here  the  air  moves  outward  (Fig.  47).  Other 
facts  to  be  noted  are,  that  the  areas  of  high  and  Ioav  press- 
ure, while  sometimes  circular  (Fig.  46),  are  more  often 
elongated  or  elliptical,  and  when  this  is  the  case,  the  long 
axis  extends  in  a  north  and  south  direction  (Fig.  48). 
In  this  case  there  is  an  even  more  notable  resemblance  to 


Fig.  48. 

Map  showing  weather  conditions  January  12,  1897.  Two  low  and  a  high- 
pressure  area  with  isobars  extending  nearly  north  and  south.  Most  intense 
shading  indicates  highest  pressure.  Temperature  in  centre  of  high,  30 
below  zero. 

a  wave,  especially  when,  as  is  sometimes  the  case,  the 
trough  of  low  pressure  covers  nearly  the  entire  Missis- 
sippi valley,  from  north  of  the  Canadian  line  to  tlie  Gulf. 
We  need  also  to  note  the  fact,  that  clouds,  rain,  and 
high  temperatures  generally  accompany  the  low-pressure 
areas,  and  that  they  always  do  so  if  these  are  strongly 


STORMS  111 

developed,  while  the  reverse  is  true  for  the  areas  of  high 
barometer. 

Origin  of  the  High-  and  Low-Pressure  Areas. — It  was  once 
believed  that  all  of  these  areas  had  their  origin  in  differ- 
ences of  temperature,  much  as  in  the  case  of  the  sea  breeze. 
Such  a  cause  would  explain  most  of  the  features  observed, 
because  warmth  would  expand  the  air,  and  inaugurate  a 
circulation,  just  as  truly  as  it  does  in  the  case  of  the  sea 
breeze,  or  the  summer  monsoon.  As  a  result  of  studies 
made  in  Europe,  doubt  has  recently  been  cast  upon  this 
theory.  If  the  air  is  warmed,  and  is  rising  in  the  low- 
pressure  areas,  the  temperature  of  the  atmosphere  on  the 
mountain  tops  in  such  a  centre  should  be  warmer  than 
that  of  the  borders ;  but  the  studies  mentioned,  which 
were  made  among  mountains,  fail  to  show  that  this  is 
true.  Also  the  facts  that  these  disturbances  of  the  air  are 
more  pronounced  in  the  coldest  parts  of  the  year,  and  that 
the  areas  of  high  and  low  pressure  pass  in  such  regular 
succession,  are  difficult  to  explain  on  this  theory.  There- 
fore, while  such  disturbances  can  be  caused  by  differences 
of  temperature,  and  while  undoubtedly  some  are,  many 
meteorologists  believe  tliat  we  must  look  for  some  other 
theory. 

No  certain  explanation  can  be  offered  in  place  of  the 
old  theory,  but  facts  point  toward  the  possible  truth  of 
the  following.  In  the  circumpolar  whirl  of  prevailing 
westerlies,  the  air  is  moving  eastward.  If  in  its  pas- 
sage it  is  thrown  into  waves,  as  the  sea  is,  and  as  we 
may  expect  it  to  be  in  passing  over  the  irregularities  of 
the  land  (Fig.  39),  troughs  and  crests  of  high  and  low 
pressure  would  be  produced.  These  should  come  in  a 
definite  order,  high  following  low,  and  with  these  varia- 


112  FIRST  BOOK  OF  PBTStCAL   GEOGRAPHY 

tions  appearing  at  frequent  intervals.  This  will  also 
account  for  the  eastward  progress  of  the  areas ;  and  which- 
ever theory  is  finally  accepted,  the  explanation  of  the 
movement  toward  the  east,  must  be  that  the  areas  move  in 
the  great  circumpolar  whirl,  and  hence  toward  the  east. 

Explanation  of  the  Winds.  —  It  has  already  been  stated 
in  sufficient  detail,  that  air  will  move  toward  areas  of  low 

BROKEN  CLOUDS  CIRRUS  CLOUDS 

."(^V,|^.^^^Jl£^^"!^^^^  ^WARMER      -^      ^ 

HEAVY  STRATUS  CLOUdJ"  »)        ^  ^AST 

DIRECTION 

Fig.  49.  °^  movement 

Diagram  showing  theoretical  movement  of  air  (by  arrows),  and  other  condi- 
tions, in  a  low  pressure  or  storm  area. 

pressure,  and  away  from  areas  of  high  barometer.  Hence 
as  such  areas  move  across  the  country,  the  winds  must 
blow  toward  a  region  of  low  barometer  and  away  from 
that  of  high,  in  the  attempt  to  equalize  the  pressure  that 
has  been  disturbed.     Since  in  the  high-pressure  areas,  air 

.•;•••■.  CIRRUS  clouds 
^•••••- ,::;-;,     _      _^    _^'^^^^^        WEATHER^      ^^^     ^ARTIY  clouds 

-STRONCS  W^INbS  ,2»,^  ^/■:.-;rJ 


'^br^^\'^^<  ^    ^  -  ,,„ 


CALM 

Fig.  50. 

Diagram  showing  theoretical  circulation  (by  arrows),  and  other  conditions,  in 
high  pressure  or  anticyclonic  area.    Temperature  rises  on  left. 

is  moving  outward  from  the  centre,  its  place  must  be 
taken  by  other  air  which  is  pushing  it  onward.  The 
source  of  this  must  be  from  above,  for  it  cannot  be  from 
either  side,  since  the  movement  is  outward  in  all  direc- 


STORMS  113 

tions.  Therefore  in  high-pressure  areas  the  air  is  settling 
from  aloft. 

In  the  low-pressure  regions,  air  moves  toward  a  centre 
which  is  constantly  shifting  its  position ;  but  as  it  comes 
from  all  sides  toward  the  centre,  some  of  it  must  find 
escape,  and  the  only  place  for  escape  is  upward.  Hence 
here,  there  is  ascending  air,  not  perhaps  of  true  convec- 
tional  origin,  but  similar  to  that  arising  from  convection. 
Perhaps  the  air  that  rises  from  the  centre  of  the  low- 
barometer  area,  passing  upward,  flows  along  toward  the 
neighboring  area  of  high  barometer,  and  there  settles, 
performing  a  journey  similar  to  that  in  the  trade-wind 
circulation  (Figs.  49  and  50). 

Uxplanatio7i  of  the  Rain.  —  The  subject  of  rain  does  not 
properly  belong  in  this  chapter,  but  on  this  point  a  few 
words  must  now  be  said.  It  is  from  the  low-pressure  areas 
that  northern  Europe  and  America  obtain  most  of  their 
rain  supply,  and  these  storms  sometimes  last  for  several 
days.  In  parts  of  New  England  these  are  called  northeast 
storms,  because  the  rainy  winds  of  the  storm  are  from  the 
east  and  northeast.  By  meteorologists  they  are  called 
cyclones,  cyclonic  storms,  or  extra-tropical  cyclones. 
The  diameter  of  the  cloudy  and  rainy  area  may  be  more 
than  1000  miles ;  and  as  the  storm  moves  eastward,  the 
entire  country,  from  the  Rocky  Mountains  to  the  Atlantic, 
and  from  the  Gulf  states  to  Hudson's  Bay,  may  receive 
rain,  or  in  winter,  snow. 

The  causes  for  these  rains  are  probably  several.  In  the 
first  place,  the  air  is  blowing  in  toward  the  low  pressure, 
and  as  it  does  so  it  is  often  obliged  to  rise  over  mountains 
or  plateaus,  or  up  the  more  moderate  grade  of  the  interior 
plains.     Since  the  temperature  of  the  atmosphere  decreases 


114  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

with  elevation  (Fig.  18),  this  lifting  of  the  air  over  the 
rising  ground  causes  it  to  cool.  Also  much  of  the  air 
moves  from  a  southern  toward  a  cooler  northern  region, 
and  in  this  way  also  its  temperature  is  lowered.  In  a 
third  way  the  air  may  become  cooled  as  it  rises  in  the  area 
of  low  pressure. 

Vapor  can  be  held  in  greater  quantities  wlien  the  tem- 
perature is  high,  than  when  low ;  and  therefore,  if  at  the 
beginning  of  its  movement  toward  the  area  of  low  barome- 
ter, the  air  was  nearly  saturated,  it  may,  by  being  cooled 
in  its  passage,  be  forced  to  give  up  some  of  its  vapor, 
forming  clouds  (Fig.  49),  and  rain  or  snow.  This  is 
particularly  liable  to  happen  if  the  winds  have  come  from 
the  ocean,  as  they  usually  have  when  they  blow  from  the 
south  and  east,  toward  the  central  and  eastern  states. 

To  explain  the  dry,  clear  air  of  the  high-pressure  areas, 
or,  as  they  are  called,  the  anticyclones^  because  of  their 
contrast  with  the  cyclones,  we  have  but  to  reverse  the 
statements  just  made.  In  these  the  air  is  settling,  and 
hence  becoming  warmer;  it  is  generally  moving  down 
grade  ;  and  it  is  often  flowing  from  cool  to  warm  regions. 
Hence  by  all  these  causes  the  anticyclonic  air  is  having 
its  temperature  raised,  and  therefore  its  capacity  to  take 
vapor  increased.  Instead  of  clouds  and  rain,  such  condi- 
tions bring  clear  and  dry  weather  (Fig.  50). 

Explanation  of  the  Temperatures.  —  When  a  cyclonic 
storm  area  is  passing  over  northern  United  States,  the 
winds  of  the  country  involved  are  usually  from  southerly 
or  easterly  quarters.  These  may  come  from  the  water, 
which  in  winter  is  warmer  than  the  land,  or  from  southern 
regions,  which  are  also  warmer. 

Hence  the  passage  of  air  toward  the  low-pressure  area 


STORMS  115 

is  a  cause  for  a  rise  in  temperature.  In  addition  to  this, 
the  cloud-covering  checks  radiation,  and  hence  prevents 
nocturnal  cooling.  Also  the  condensation  of  vapor  is  a 
warming  process,  the  so-called  latent  heat,  or  the  heat  that 
is  expended  in  transforming  the  water  to  vapor,  is  liber- 
ated when  the  clouds  form,  and  raindrops  are  produced 
(p.  62).  Hence  even  when  a  winter  storm  begins  as  a 
cold  snowstorm,  if  the  condensation  of  vapor  continues, 
in  time  the  weather  moderates,  and  perhaps  the  snow- 
storm may  end  in  rain.  It  is  also  true  that  this  liberated 
heat  furnishes  energy  to  the  storm,  warming  the  air  and 
making  it  lighter,  thus  decreasing  the  pressure  and  in- 
creasing the  wind  velocity  and  rainfall  as  well  as  the 
general  intensity  of  tlie  storm.  The  storm  has  become 
a  great  engine,  which  once  started  increases  by  the  aid 
of  fuel  which  it  supplies  to  itself. 

The  coolness  of  the  high-pressure  anticyclone,  which 
in  winter  may  produce  a  cold  wave,  and  spread  a  blanket 
of  ice-cold  air  over  the  land,  is  also  due  to  several  causes. 
The  settling  of  the  air  from  aloft  brings  down  to  the  earth 
the  low  temperatures  of  the  upper  atmosphere  (Fig.  50). 
Since  the  centre  of  the  anticyclone  is  generally  in  the  nortli, 
the  air  that  flows  over  the  United  States  usually  comes 
fi'om  northerly  quarters,  and  hence  from  colder  lands. 
Since  this  air  is  dry  and  cool,  radiation,  both  of  day  and 
night,  proceeds  with  rapidity;  and  as  a  result  of  these 
several  causes,  the  anticyclone  is  distinctly  cooler  than 
the  low-pressure  area.  When  a  well-developed  anti- 
cyclone overspreads  the  northern  states,  the  rapid  radia- 
tion of  night-time  may  cause  even  the  summer  night  to  be 
uncomfortably  cool,  while  in  spring,  late  frosts  may  come, 
or  in  the  fall,  vegetation  may  suffer  from  an  early  frost. 


116 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Hurricanes  or  Tropical  Cyclones :  Time  and  Place  of 
Occurrence.  —  The  sailors  of  the  Atlantic  believe  that  a 
violent  storm  may  be  expected  at  the  autumn  equinox, 
near  the  middle  of  September;  and  in  reality,  during 
August,  September,  and  early  October,  the  western 
Atlantic  is  liable  to  be  visited  by  one  or  several  very 
violent  storms,  which  are  called  hurricanes  or  tropical 
cyclones.  Similar  storms  visit  the  Indian  Ocean  and  the 
south  Pacific.     The  typhoon^  which  quite  often  devastates 


Fig.  51. 

A  tropical  cyclone  in  India,  showing  spirally  inflowing  winds  toward  area  of 

low  pressure. 

the  Asiatic  coast,  is  a  similar  disturbance  of  the  air.  Such 
storms  are  not  known  in  the  south  Atlantic,  nor  do  they 
ever  originate  on  the  land.  They  come  in  the  autumn 
months  (the  autumn  corresponding  with  our  spring  in  the 
southern  hemisphere),  and  are  practically  confined  to  these. 
Their  birthplace  is  near  the  tropics  over  the  ocean. 

The  West  Indian  hurricanes  have  their  origin  between 


FacAng  page  116. 


Plate  10. 


i.D.fjrw./ jv.r.', )    >  ,' ^, 


A  tropical  cyclone  or  West  Indian  hurricane.  Path  shown  by  line  of  arrows 
upon  which  the  dates  of  passage  are  indicated .  Winds,  isobars,  and  isotherms 
shown.    Rain  indicated  by  shading.    Path  somewhat  abnormal. 


Facing  pane  m.  Platic  11.  "'"''' ''''' 

Map  of  tyi)ioal  winter  storm  to  show  difference  in  temperature  in  different 
parts.  Over  dotted  area  rain  is  falling,  over  cross-shaded  portion,  near 
boundary  between  cyclone  and  anticyclone,  snow  is  falling. 


STORMS 


117 


Florida  and  the  South  American  coast,  from  which  they 
move  northward,  with  the  centre  generally  off  our  coast 
(Fig.  45),  usually  causing  terrific  gales  from  Texas  to 
Nova  Scotia.  After  passing  along  this  coast,  they  cross 
the  Atlantic,  approximately  along  the  path  pursued  by 
the  cyclones,  turning  toward  the  east,  under  the  influence 
of  rotation,  in  exactly  the  same  way  that  the  trade  winds 
are  deflected.  Sometimes  these  storms  diverge  from  this 
path  and  pass  over  the  land,  so  that  the  centre  moves 
over  the  eastern  states  (Plate  10). 

Characteristics.  —  Where  first  noticed  as  distinct  storms, 
the  hurricanes   are    disturbances   much   smaller  in    area 


INCHES 

30.4 
30.3 
.30.2 
30.1 
30.0 
20.9 
29.8 
29.7 
29.C 
29.5 
29.4 
29.3 
29.2 
29.1 
20.0 

SEPT.   29.    1836. 

ISEPT.    30,    1896. 

OCTOBER   1 

OCTOBER  2 

OCT.    3,    1896.     1 

NOON 

NOON 

NOON 

NOON 

NO.ON             1 

^ 

/ 

■          ^ 

\ 

/^^^ 

\ 

/- 

\ 

/ 

\ 

_y 

/ 

r 

J- 

I 

1 

y 

Fig.  52. 

Diagram  showing  sudden  fall  of  barometer  at  Ithaca,  N.Y.,  during  the  pas- 
sage of  a  hurricane,  the  winds  of  which  did  much  damage. 

than  the  cyclonic  storms  which  we  have  been  considering. 
In  the  centre  the  pressure  is  very  low,  and  the  isobars  are 
crowded  together,  so  that  in  a  short  distance  the  pressure 
may  change  more  than  an  inch.  Toward  this  centre,  the 
air  goes  from  all  sides  with  great  force  (blowing  60  or  70 


118  FIBST  BOOK  OF  PHYSICAL   GEOGBAPHY 

miles  an  hour),  turning  spirally  as  it  moves,  somewhat 
as  water  does  in  escaping  from  a  basin.  Exactly  in  the 
centre,  the  air  is  rising  vertically,  and  there  the  sky  may 
be  clear,  while  all  around  it  are  clouds,  from  which  tor- 
rents of  rain  are  falling.  A  vessel  that  has  the  misfortune 
to  come  within  the  reach  of  the  more  violent  part  of  the 
hurricane,  if  it  escapes  at  all,  does  so  only  after  suffering 
much  damage.  Seacoast  towns  over  which  hurricanes 
pass,  are  often  devastated,  and  this  forms  one  of  the  most 
violent  and  destructive  classes  of  storms.  As  they  come 
out  of  the  tropics,  they  gradually  lose  violence,  and  gen- 
erally at  the  same  time  increase  in  area. 

Explanation.  —  The  origin  of  hurricanes  seems  evident  from  the 
fact  that  they  always  begin  in  warm  regions.  This  points  to  convec- 
tion as  their  cause.  Unusual  heat  brings  about  conditions  as  a  result 
of  which  air  must  rise,  and  hence,  upon  cooling,  furnish  rain.  The 
heat  thus  liberated  by  the  condensed  vapor,  increases  the  ascent  of 
the  air  by  warming  it,  and  this  decreases  the  already  low  pressure. 
Toward  this  area  of  ascent,  air  comes  from  all  sides,  forming  winds, 
which  move  spirally  because  they  are  deflected  by  the  effect  of  rota- 
tion. The  storm  increases,  slowly  moves,  and  finally,  passing  into 
the  cooler  regions,  loses  intensity;  for  the  cooler  air  that  exists  in 
the  more  northern  latitudes,  contains  less  vapor  to  supply  the  heat 
energy  with  which  the  intensity  of  the  tropical  storm  is  maintained. 
Such  a  storm  could  not  be  formed  over  the  land,  because  the  air  is  nol 
so  humid  as  over  the  water;  and  hence  the  heat  caused  by  the  con 
densation  of  vapor  could  not  be  supplied  in  such  amount. 

But  much  heat  reaches  the  tropical  zone  at  all  times  of  the  year  ; 
and  why  then  do  we  not  have  such  storms  at  all  times?  and  why  are 
none  found  in  the  south  Atlantic?  To  explain  these  two  peculiari- 
ties, we  must  recall  the  fact  that  the  deflective  influence  of  rotation, 
which  gives  rise  to  the  whirling  movement  of  the  air,  decreases  from 
polar  to  tropical  latitudes,  and  near  the  Equator  is  so  slight,  that 
the  air  currents  in  the  belt  of  calms  cannot  be  turned  very  decidedly 
to  one  side.     In  the  autumn  the  belt  of  greatest  heat  is  furthest  from 


STORMS 


im 


the  Equator,  and  hence  nearest  the  region  where  this  deflective  effect 
can  produce  decided  influence  upon  the  direction  of  the  winds.  Hence 
at  this  time  only,  can  the  winds  which  blow  toward  the  region  of  low 
pressure  of  the  heated  belt,  start  whirling  ;  and  without  the  whirl  the 
storm  cannot  exist.  Since  this  heated  belt  never  goes  far  south  of  the 
Equator  in  the  south  Atlantic,  such  storms  cannot  visit  this  ocean. 


«•          «0OH                  M                .OOH                 „                .00.                 „                «00«                 «                HOC-                 M                 .OOH                M                .OO.                  I 

50 

45 
40 

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February  25, 1895 

26                 1                27 

28 

Maroni 

2 

1 

Fig.  53. 

Temperature  record  for  several  successive  days  (Ithaca,  N.Y.),  showing  effect 
of  two  cold  waves,  and  (in  centre)  of  Sirocco,  which  brought  a  high  tem- 
perature for  several  days. 


Storm  Winds.  —  While  the  greater  part  of  the  United 
States  is  within  the  belt  of  prevailing  westerlies  (Chapter 
VII),  so  that  west  winds  prevail,  the  winds  near  the  sur- 
face are  mainly  determined  by  the  passage  of  the  high- 
and  low-pressure  areas.  The  hot  summer  winds,  which 
come  from  the  south,  generally  represent  air  that  is  slowly 
flowing  in  toward  a  low-pressure  area  in  the  far  north. 
They  are  often  muggy  winds  because  their  source  is  from 
the  warm,  humid  sea.  Sometimes  these  winds  blow  day 
after  day  without  cessation,  and  then  the  land  may  be 
visited  by  a  summer  drought.  During  the  winter  our 
warm  winds  are  also  from  the  southern  quarter,  and  these 
likewise  are  moving  toward  a  storm  centre  (Figs.  53  and  54). 
It  is  such  winds  as  these  which  cause  the  winter  thaws. 


120 


FIRST  BOOK  OF  PHTSICAL   GEOGRAPHY 


In  Europe  similar  warm  Avinds  are  called  siroccos^  and  this 
name  may  also  be  applied  to  our  own  warm  south  winds. 

During  the  progress  of  a  storm  the  wind  in  any  particular 
place  may  veer  through  various  quarters :  the  warm  south 
wind  may  be  followed  by  a  warm  and  rainy  southwest  or 
southeast  wind,  and  this  by  wind  from  the  east,  which 
bears  rain,  and  this  in  turn  by  cold  air  moving  from  the 
northwest.  The  latter  illustrates  an  exactly  opposite  type 
from  that  of  the  sirocco  (Figs.  53  and  54).  On  the  rear 
or  west  side  of  a  storm,  there  are  often  strong  and  even 
fierce  west  and  northwest  winds,  perhaps  accompanied  by 
snow  (Plate  11).     They  represent  cold  air  coming  partly 


ES 


^^' 


m 


Fig.  54. 

Temperature  record  (Ithaca,  N.Y.)  showing  how  for  thirteen  successive  days 
the  daily  temperature  range  was  destroyed  by  cyclones  and  anticyclones. 

from  the  upper  layers  of  the  atmosphere,  and  partly  from 
the  cool  interior  and  more  northerly  regions.  In  Texas 
such  a  wind  is  called  a  norther^  in  Dakota  a  blizzard. 

Milder  types  of  blizzards  occur  in  New  York,  and  other 
eastern  states,  after  many  of  the  winter  storms.  During 
such  a  time,  perhaps  after  a  rain  storm,  as  the  wind 
changes  the  temperature  descends,  perhaps  even  at  mid- 
day, cold  snow  squalls  occur,  and  soon  the  thermometer 
has  fallen  nearly  to  zero.  After  this  comes  the  calm  of 
the  anticyclone,  and  the  land,  already  covered  with  a 
blanket  of  cold,  clear  air,  cools  still  more  by  radiation, 
]intil  even  Jower  temperatures  occur.     The  cool,  dry,  and 


STOBMS 


121 


refreshing  west  wind  of  summer,  which  succeeds  the  sultry 
weather  that  has  perhaps  terminated  in  a  thunder  shower, 
is  the  summer  equivalent  of  the  winter  cold  wave. 

Sometimes  in  the  west,  near  the  eastern  base  of  the  Rocky  Moun- 
tains, in  Montana  and  elsewhere,  a  wonderfully  dry  and  warm  wind 
springs  up  from  the  west,  perhaps  causing  all  the  snow  to  disappear 
from  the  ground.     This  wind  is  known  as  the  chinook  (Fig.  55),  and 


1892 

J»n.  ir 

Jan.  18 

Jan.  19 

J«n.  20 

Jan.  2t 

1892 

...-,           <2        .P.M. 

,.-.           ,.        ,P.M.          1 

..«.  .,  .p.-. 

e».M.      u     «P.M. 

.».H.            ,.       ,P.M.                1 

\\ 

/ 

^ 

\ 

rs^  ", 

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^ 

/ 

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V 

/ 

\ 

A./ 

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10 

10 

" 

10° 

"^ 

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y 

^\^ 

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, 

, 

Fig.  55. 

Temperature  records  during  the  blowing  of  the  Chinook  wind  in  Montana. 
In  less  than  an  hour  the  thermometer  rose  from  10°  below  zero  nearly  to 
40°  above  —  a  rise  of  nearly  50^. 

a  wind  of  the  same  kind  is  known  in  Switzerland  as  the  Foehn.  Also 
in  Greenland,  the  air  which  flows  down  from  the  great  ice  and  snow 
covered  interior,  is  sometimes  warm  instead  of  cold,  as  we  should  ex- 
pect. These  warm  winds  are  caused  by  air  blowing  down  the  moun- 
tain slopes  toward  a  centre  of  low  pressure.  When  air  settles  rapidly, 
its  temperature  rises  as  a  result  of  the  compression,  and  hence  it 
reaches  the  ground  much  warmer  than  when  it  started.^  The  air  is 
made  dry,  because  as  the  temperature  rises,  its  ability  to  carry  vapor 
is  increased. 

Thunder  Storms.  — During  the  summer,  after  an  oppres- 
sive day,  we  look  for  a  thunder  storm,  and  it  frequently 

iThis  is  a  fact  of  physics,  and  is  merely  stated  here  as  a  fact,  —  that 
descending  air  has  its  temperature  increased,  and  ascending  air  is  cooled 
as  it  rises  and  expands. 


122  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

comes ;  and  often  when  one  does  not  visit  us,  they  occur 
to  the  north  or  south,  and  even  sometimes  so  near  that  we 
see  the  lightning  and  even  hear  the  thunder.  In  northern 
United  States  these  storms  come  from  the  west,  usually  in 
the  afternoon  or  early  evening.     Following  the  storm  the 


Fig.  56. 
Photograph  of  a  distant  thunder  storm. 

air  is  generally  fresher,  and  cool,  dry  west  winds  replace 
the  sultry  air  that  has  been  coming  from  the  south. 

Thunder  storms  are  of  frequent  Occurrence  within  the 
tropical  belt.  As  the  air  is  warmed  by  the  morning  sun, 
great  banks  of  cloud  begin  to  develop  overhead,  and  finally 
rain  falls,  while  lightning  and  thunder  appear.  Here  the 
cause  is  evidently  the  rise  of  the  air  by  convection ;  for 
rising  air  cools,  and  if  it  started  with  much  vapor,  some  is 
condensed  to  form  clouds,  and  later  rain,  because  with  a 
lower  temperature  some  of   the  vapor  must  be  given  up. 


STORMS 


123 


Around  mountain  peaks  similar  storms  are  developed  in 
summer,  and  here  also  convection  is  the  cause. 

The  same  explanation  appears  to  apply  to  the  thunder 
storms  of  the  United  States,  for  these  come  only  in  the 
warm  months,  and  when 
the  air  is  very  humid. 
Moreover,  their  coming 
is  preceded  by  the  for- 
mation of  cloud  banks, 
similar  to  those  caused 
by  convection  in  the 
tropics.^  If  one  will  ex- 
amine the  weather  map 
for  a  day  when  thunder 
storms  occur,  he  will 
find  that  they  have  de- 
veloped in  the  southern 
quarter  of  a  low-press- 
ure area,  where  warm, 
humid  south  and  south- 
east winds  are  blowing 
toward  the  storm  centre 
(Fig.  57).  They  are 
therefore  secondare/ 
storms,  occurring  in  a 
larger  area  of  low  pressure,  and  developed  mainly  because 
of   the   heat   made   possible   by  the  south  winds,  and  of 


Fig.  57. 

Part  of  weather  map  July  16, 1892,  showing 
storm  over  eastern  Canada  and  north- 
ern New  England.  Thunder  storms 
occurred  at  places  marked  by  arrows. 


1  That  convection  is  the  cause  for  these  clouds  is  shown  by  the  fact 
that  they  often  form  over  the  land,  and  not  over  the  cooler  ocean,  where 
there  is  less  convection.  In  sailing  off  shore,  one  often  sees  a  line  of  these 
clouds  while  the  sky  overhead  is  clear ;  and  the  presence  of  distant  land, 
which  is  out  of  sight,  is  shown  by  these  banks  of  clouds. 


124  FIRST  BOOK  OF  PHYSICAL    GEOGRAPHY 

the  moisture,  which  is  due  to  the  same  cause.  They  move 
eastward  in  the  same  direction  as  the  low-pressure  area, 
and  sometimes  many  such  storms  develop  and  progress 
eastward,  following  approximately  parallel  paths.  Some 
such  storms  have  travelled  from  New  York  across  New 
England,  and  disappeared  upon  passing  out  to  sea. 

Tornadoes.  —  Fortunately  these  terrible  storms  are  uncommon  in 
most  of  the  country,  and  where  they  do  occur,  they  extend  over  only 
a  very  small  tract.  As  in  the  case  of  thunder  storms,  they  develop  in 
the  southern  part  of  low-pressure  areas,  and  move  eastward  during 
the  afternoon  or  evening  of  hot,  muggy  days,  generally  in  summer. 
Seen  from  one  side,  they  consist  of  a  spout  of  black  cloud,  spreading 
out  into  an  umbrella  shape  above  (Fig.  58).  Rain  and  hail  fall  from 
the  margin,  and  lightning  and  thunder  accompany  the  storm.  Ex- 
cepting near  the  spout  the  wind  is  not  violent ;  but  in  this  it  attains 
such  a  velocity  that  strong  buildings  are  torn  apart,  trees  uprooted, 
heavy  objects  lifted  and  borne  away,  and  many  remarkable  feats  per- 
formed. 

The  wind  in  a  tornado  blows  spirally  toward  the  centre  of  the 
spout,  with  increasing  velocity  until  the  centre  is  reached,  where  the 
air  is  rising  rapidly  enough  to  lift  the  roofs  of  houses  from  their  sup- 
ports. Here  no  rain  can  fall,  for  everything  so  light  must  rise.  In 
the  centre  the  barometric  pressure  is  extremely  low,  and  the  condition 
of  a  vacuum  is  so  nearly  reached,  that  the  expansion  of  the  air  within 
the  houses  sometimes  blows  the  walls  outward.  The  tornado  spout  is 
somewhat  like  the  whirl  of  water  which  escapes  from  the  outlet  of  a 
wash  basin.  On  a  small  but  much  more  violent  scale  it  is  like  a  hur- 
ricane, and  on  a  much  larger,  and  also  more  violent  scale  it  resembles 
the  tiny  dust  whirls  of  the  desert,  which  I  shall  describe. 

On  a  plain,  and  better  still  on  deserts,  the  heat  of  the  sun  warms 
the  air  near  the  ground,  until  its  temperature  is  several  degrees  higher 
than  the  layers  above.  This  is  an  unstable  and  unnatural  condition, 
for  warm,  light  air  should  rise;  but  the  day  is  so  quiet  that  for  awhile 
nothing  causes  it  to  start.  Then  perhaps  the  flutter  of  a  bird  starts  a 
movement  which  shall  give  relief  to  the  unnatural  arrangement  of  air 
layers.     Air  presses  from  all  sides  toward  the  place  where  the  ascent 


STORMS 


125 


is  being  begun,  and  soon  a  slight  whirl  is  started,  and  the  movement 
of  the  air  is  so  rapid  that  the  winds  carry  dust,  leaves,  and  even  sticks, 
which  in  the  centre  rise,  and  spreading  out  above,  fall  to  the  ground 
on  one  side  of  the  centre.  From  all  directions  the  air  moves  toward 
this  spout,  and  the  little  dust  whirl  itself  moves  slowly  across  the  plain, 
so  that  if  one  should  stand  in  its  path,  he  would  find  the  wind  first  in 
his  back,  when  his  hat  would  rise  into  the  air,  and  quickly  the  wind 


Ik     m 

1_.^-- 

;^'^-iRi£^;.^                  <-.■  ■  -'' 

Fig.  58. 
A  tornado  near  St.  Paul,  Minnesota,  July  13, 1890. 


would  blow  directly  in  his  face,  at  first  briskly,  then  more  gently, 
until  finally  replaced  by  the  calm  of  the  desert.  On  a  milder  and 
very  small  scale  this  is  what  is  experienced  in  a  tornado. 

During  days  when  tornadoes  come,  the  air  near  the  ground  is  very 
warm  and  humid,  while  cooler  layers  of  air  exist  above.  Convection 
causes  a  whirl  to  start,  and  a  tornado  develops,  being  no  doubt  in- 
creased in  force  by  the  formation  of  rain,  which  causes  more  heat. 
The  reason  why  tornadoes  are  more  abundant  in  the  Mississippi  val- 
ley than  elsewhere,  is  that  here,  over  the  great  plains,  the  warm  air 
from  the  south  is  more  easily  drawn  in  under  the  cool  air,  which  is 
moving  from  the  west  in  the  prevailing  westerlies. 


CHAPTER  IX 

MOISTURE   IN   THE  ATMOSPHERE 

Vapor.  —  This  invisible  form  of  water  is  always  present 
in  the  air,  and  every  now  and  then  some  of  it  is  being 
changed  from  an  invisible  gas  to  the  liquid  or  solid  form 
of  water.  This  substance  finds  its  way  into  the  air  as 
the  result  of  evaporation^  and  at  nearly  all  times  vapor  is 
being  taken  from  all  water  bodies  in  the  world,  as  it  is 
also  from  damp  ground  and  from  the  leaves  of  plants.  It 
is  possible  for  some  vapor  to  be  held  in  all  air^  no  matter 
how  cold,  but  there  is  a  limit  to  the  amount  that  air  of 
any  given  temperature  can  hold.  When  the  air  has  so 
much  vapor  that  no  more  can  be  taken,  it  is  said  to  be 
saturated;  hence  such  humid  or  saturated  air  does  not 
have  the  power  to  carry  on  evaporation  further.  On  the 
other  hand,  an  air  that  is  dry,  and  not  nearly  saturated,  is 
capable  of  rapid  evaporation.  Therefore  in  deserts,  where 
the  air  is  exceedingly  dry,  water  cannot  stand  long  with- 
out being  evaporated.     . 

The  rate  of  evaporation  depends  partly  upon  the  dryness 
of  the  air,  partly  on  its  temperature,  and  partly  on  its 
movement.  When  the  air  remains  quiet  over  a  pond,  it 
may  become  saturated,  and  hence  for  the  time  being, 
evaporation  over  the  water  surface  may  be  checked;  but 
if  the  wind  is  blowing,  there  are  constant  supplies  of  new 

126 


MOISTURE  IN   THE  ATMOSPHERE  127 

a^V,  and  hence  evaporation  proceeds  without  interruption. 
The  winds  that  pass  over  the  great  ocean  can  obtain  much 
vapor,  and  hence  the  air  of  oceans,  as  well  as  that  of  the 
land  near  them,  is  generally  more  humid  than  that  far 
away  in  the  interior  of  continents. 

In  any  given  amount  of  air  there  is  always  a  certain 
quantity  of  water  vapor ;  and  this  is  known  as  the  absolute 
humidity^  and  could  be  measured  in  pounds  (Chapter  III). 
If  we  suppose  that  air  with  a  temperature  of  60°  has  |  as 
much  vapor  as  it  could  possibly  contain  at  that  tempera- 
ture, this  1^  would  be  called  the  relative  humidity ;  that  is,  it 
would  be  the  percentage  of  vapor  actually  present  in  the  air, 
compared  with  that  which  might  he  held  at  that  temperature. 
If  it  contained  all  that  could  possibly  be  held,  or  was 
saturated,  the  relative  humidity  would  be  100% ;  hence 
the  |,  which  we  have  supposed,  would  be  75%  of  the  pos- 
sible, and  the  relative  humidity  would  therefore  be  75%. 
Should  the  temperature  of  this  same  air  fall,  even  without 
the  least  change  in  the  real  amount  of  vapor,  or  the  abso- 
lute humidity,  the  relative  humidity  would  be  increased, 
because  cold  air  is  able  to  carry  less  vapor  than  when 
warmer.  Indeed,  it  is  possible  that  the  temperature  may 
descend  until  the  point  of  saturation  is  reached,  and  if 
this  be  so,  some  of  the  vapor  must  be  given  up  either  in 
the  form  of  fog,  rain,  dew,  frost,  snow,  or  hail.  This 
point  of  saturation,  when  the  relative  humidity  stands  at 
100%,  is  known  as  the  dew  point. 

If,  instead  of  falling,  the  air  temperature  rises,  the  rela- 
tive humidity  will  decrease,  because  it  can  hold  more  vapor 
than  formerly;  and  therefore,  with  a  higher  temperature, 
the  percentage  of  that  held  compared  with  what  might  be 
carried  is  smaller.     These  facts  have  important  bearings 


128 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


on  the  explanation  of  the  precipitation  of  moisture  from 
the  air.  Evaporated  and  transformed  to  a  gas  which  no 
one  can  see,  the  vapor  passes  hither  and  thither,  until 
finally,  by  some  change  of  temperature,  it  can  no  longer 
exist  as  vapor,  but  must  assume  its  old  form,  and  perhaps 
return  to  the  very  ocean  which  gave  it  birth. 

Almost  any  place  in  moist  countries  offers  frequent 
illustrations  of  this  change  in  relative  humidity.     Per- 


MONDAY 

TUESDAY 

WEDNESDAY 

THURSDAY 

FRIDAY 

SATURDAY 

SUNDAY 

6         XII        6 

6           XII         6 

6          XII       6 

6           XII          6 

6        XII        6 

6          XII         6 

6         XII        0 

/ 

n 

/ 

^ 

rn 

A 

^ 

/ 

\ 

/ 

^ 

/ 

\ 

/^ 

J 

V 

\ 

J 

V 

\  ) 

\ 

\/ 

•n 

y 

\ 

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I 

/ 

w 

sj 

J 

AUG.  14 


AUG.  20,  18 


RELATIVE   HUMIDITY 
iTHACA 

Fig.  59. 


Record  of  relative  humidity  for  a  week  at  Ithaca,  N.Y.  Nearly  every  night 
the  dew  point  is  reached,  but  at  midday  the  relative  humidity  is  only  from 
30''-60^ 


haps  for  several  days  the  air  of  a  place  has  about  the  same 
actual  amount  of  vapor,  or  the  same  absolute  humidity, 
but  the  temperature  of  the  air  varies  from  day  to  night. 
As  the  sun's  heat  warms  the  earth  in  the  daytime,  the 
relative  humidity  of  the  air  decreases,  and  perhaps  by 
noon  there  is  only  50%  as  much  vapor  as  can  be  held  at 
that  temperature,  and  then  evaporation  is  rapid,  as  may 
be  seen  by  the  fact  that  clothes  on  the  line  dry  quickly. 
In  the  afternoon  the  temperature  descends,  and  therefore 
the  relative  humidity  rises,  until  perhaps  the  point  of 


MOISTURE  IN   THE  ATMOSPHERE 


129 


saturation  is  reached  (Fig.  59),  and  then  dew 
may  form;  and  if  clothes  have  been  left  out 
upon  the  line,  they  again  begin  to  become 
damp,  although  before  sunset  they  were  dry. 
In  addition  to  this  daUi/  change,  dependent 
entirely  upon  variation  in  temperature,  there 
are  also  changes  in  the  absolute  humidity  of 
the  air,  for  some  winds  are  damp  and  others 
dry. 

Instruments  for  Measuring  Vapor.  —  Although  we 
cannot  see  vapor,  there  are  various  M^ays  in  which  we 
can  measure  the  relative  humidity.  One  of  the  commonest 
of  these  is  the  hair  hrjgrometer,  which  consists  of  a  bundle  of 
hair  robbed  of  its  oil.  The  individual  hairs  absorb  the 
vapor  in  proportion  to  its  amount,  and  as  they  absorb  or 
give  up  vapor  they  lengthen  or  contract.  This  movement 
of  the  hairs  can  be  made  to  move  a  hand  across  a  graduated 
scale,  so  that  readings  of  the  length  may  be  made,  and  from 
this  the  relative  humidity  be  calculated.  The  operation  of 
this  instrument  is  upon  the  same  principle  that  causes  hair 
not  naturally  curly  to  lose  the  artificial  curl  when  exposed 
to  damp  air.  This  results  from  the  absorption  of  vapor 
from  the  air. 

Another  means  for  making  this  measurement  is  by  the 
use  of  two  thermometers,  one  having  its  bulb  encased  in  a 
piece  of  wet  muslin.  This  instrument,  which  is  called  a 
psi/cJirometer,  is  whirled  in  the  air,  and  one  of  the  thermom- 
eters records  the  real  air  temperature,  while  the  other,  which 
has  its  bulb  wrapped  in  wet  muslin,  records  a  lower  temper- 
ature, because  the  evaporation  of  water  from  the  muslin 
produces  cold,  as   evaporation   always   does.^    If  the  air  is 

Fig.  60. 


Psychrometer  or 
dry  (right  hand) 
and  wet  (left 
hand)  bulb  ther- 
mometer. 


1  This  principle  is  made  use  of  in  dry  coun- 
tries to  keep  water  cool.  Water  in  a  porous  jar 
evaporates  through  the  sides,  thus  cooling  the  jar 
and  also  the  water.  In  travelling  in  such  coun- 
tries it  is  customary  to  carry  a  canteen  of   tin, 


130  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

very  dry,  the  evaporatic^  is  rapid,  if  it  is  humid,  the  evaporation  pro- 
ceeds slowly;  and  hence  in  the  former  case  the  difference  in  temper- 
ature recorded  by  the  two  instruments,  is  greater  than  in  the  latter. 
By  means  of  tables  made  for  the  purpose  (these  may  be  obtained 
from  the  U.  S.  Weather  Bureau  at  Washington)  the  relative  humid- 
ity may  be  calculated,  and  from  them  also  it  is  possible  to  tell  at 
just  what  temperature  the  dew  point  will  be  reached  under  all  con- 
ditions of  relative  humidity.  ^ 

The  rate  of  evaporation  is  generally  determined  by  means  of  a  pan 
of  water  (called  an  evaporating  dish)  either  placed  upon  scales,  and 
hence  weighed,  or  else  one  in  which  there  is  a  graduated  rod.  By 
means  of  this  rod  the  measurement  of  the  depth  of  water  evaporated 
is  made  in  inches ;  and  hence,  in  stating  the  evaporation,  it  is  custo- 
mary to  say  that  it  amounts  to  so  many  inches  in  a  year,  by  this 
meaning  in  the  course  of  a  year  that  evaporation  from  a  water  body 
would  lower  it  just  that  number  of  inches.  In  dry  regions,  particularly 
in  deserts,  the  amount  of  evaporation  exceeds  that  of  the  rainfall,  and 
the  soil  is  kept  constantly  dry,  because  the  air  is  always  greedy  for 
more  moisture  than  it  can  find. 

Dew.  —  During  the  summer,  and  at  other  times  when 
the  temperature  does  not  fall  below  the  freezing  point,  the 
setting  of  the  sun  is  often  followed  by  an  increasing  damp- 
ness of  the  grass  and  other  objects  that  are  near  the  ground. 
This  dampness  we  call  dew^  and  it  may  form,  or  "fall," 
even  before  the  sun  has  finally  set;  or  possibly  its  forma- 
tion may  not  begin  until  late  at  night,  and  during  some 
nights  no  dew  forms,  especially  if  the  sky  is  cloudy. 
Sometimes  dew  gathers  only  in  certain  places,  and  again, 

covered  with  a  woollen  cloth.  So  long  as  the  cloth  is  kept  damp,  the 
evaporation  of  water  from  the  surface  keeps  the  canteen  and  its  contents 
cool,  even  though  exposed  to  the  direct  rays  of  the  sun.  In  dry  countries 
one  may,  therefore,  have  cool  drinking  water  even  on  the  hottest  day. 

1  There  is  a  similar  instrument  which  is  not  whirled,  but  kept  station- 
ary. In  this  a  wick  leading  from  a  disli  of  water  keeps  the  muslin  damp, 
the  water  rising  as  oil  does  through  a  lamp  wick. 


MOtSTUttE  IN   THU  ATMOSPHERE  131 

particularly  after  a  muggy  summer  day,  so  much  gathers 
that  all  vegetation  is  dripping  wet,  as  if  with  rain. 
Shortly  after  sunrise  the  glittering  drops  of  dew  disap- 
pear, being  evaporated  under  the  warming  influence  of 
the  sun. 

The  production  of  dew  depends  upon  the  change  in 
relative  humidity.  The  air,  perhaps  very  humid,  as  is 
sometimes  the  case  in  summer,  is  cooled  by  radiation  after 
the  sun  sinks  in  the  west;  soon  the  dew  point  is  reached 
(Fig.  59),  and  from  the  then  saturated  air,  some  vapor 
passes  into  the  form  of  liquid  water,  just  as  vapor  in  a  room 
may  condense  on  the  surface  of  the  cold  window.  Those 
objects  that  have  cooled  most,  receive  the  greatest  supply 
of  dew,  and  vegetation,  which  is  one  of  the  best  of  radia- 
tors, is  most  abundantly  supplied.^  With  many  clouds  in 
the  sky,  radiation  is  checked,  and  the  dew  point  may  not 
be  reached;  and  also  if  the  air  is  very  dry,  the  cooling  at 
night  may  not  go  far  enough  to  reach  the  dew  point,  or 
perhaps  only  just  far  enough  for  a  little  to  collect.  Silently 
it  accumulates,  not  by  falling^  but  by  condensation  from  the 
air  upon  the  surface  of  those  objects  that  are  coolest. 

Probably  this  cause  for  dew  formation  is  aided  by  another,  which 
also  results  from  radiation.  At  all  times  vapor  is  being  exuded  from 
the  damp  ground,  and  particularly  from  plants,  in  which  it  rises  from 
the  ground  in  the  form  of  sap.  During  the  daytime  this  is  evaporated, 
and  does  not  appear  in  the  form  of  drops  of  water  which  are  visible ; 
but  at  night,  when  the  air  is  nearly  or  quite  saturated,  evaporation 
cannot  proceed,  and  the  dampness  accumulates  on  the  surface  of  the 
ground  and  plants,  adding  to  the  quantity  that  comes  from  the  air. 
This  is  why  dew  gathers  on  the  under  side  of  leaves  and  other  objects 
spread  out  near  the  ground. 

1  This  is  one  of  the  many  beautiful  adjustments  of  nature,  by  which 
animals  and  plants  make  use  of  Nature's  riches. 


132  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Frost.  —  This  condensation  of  vapor  from  the  air  pro- 
daces  frost  whenever  the  temperature  of  the  dew  point  is 
32°  or  less.  Frost  is  not  frozen  dew,  but  merely  the  solid 
form  assumed  by  the  condensation  of  invisible  vapor,  at  a 
temperature  below  the  freezing  point.  It  is  quite  like  the 
formation  of  frost  work  on  the  window,  where  the  frost 
crystals  may  be  seen  to  grow  as  solid  forms,  without  an}' 
previous  deposit  of  liquid  water  which  can  freeze.  There- 
fore the  remarks  that  have  been  made  about  dew  apply 
quite  fully  to  frost. 

There  are  many  peculiarities  in  the  distribution  of  frost. 
Sometimes  it  is  very  heavy,  and  the  grass  and  earth  are 
white  with  it,  but  at  other  times  there  is  only  a  very  light 
frost  in  a  few  places.  In  the  latter  case  very  slight  differ- 
ences in  exposure,  or  in  the  nature  of  the  ground,  will 
cause  differences  in  amount;  and  indeed  in  one  place  dew 
may  accumulate,  Avhile  in  others  frost  gathers.  The  smoke 
of  a  fire,  or  a  cloth  spread  over  a  plant,  will  often  prevent 
frost  by  checking  radiation,  and  thus  the  cooling.  Low 
ground  is  visited  earlier  than  the  higher  land,  particularly 
if  the  lower  ground  is  damp ;  for  then  there  is  more  vapor 
in  the  air.  It  is  partly  for  this  reason  that  in  autumn 
the  leaves  of  trees  in  swamps  turn  earlier  than  those  on 
the  dry  hillsides.  There  is  another  cause  besides  this  one ; 
for  as  radiation  proceeds,  and  the  air  near  the  ground 
cools,  it  becomes  heavy  and  slides  down  the  hillsides, 
thus  causing  movement  and  a  stirring  of  the  air,  while  in 
the  valleys  the  cold  layers  settle  and  remain  much  more 
quiet.  Moving  air  is  not  easily  cooled,  because  as  soon 
as  the  radiation  from  the  ground  has  lowered  the  tem- 
perature of  the  air  near  it,  it  slides  away,  and  other  layers 
take  its  place. 


MOISTURE  IN   THE  ATMOSPHERE  133 

Fog.  —  Sometimes,  particularly  in  damp  places,  the 
cooling  of  the  ground  by  radiation  chills  the  air  for  some 
distance  above  it,  and  lowers  its  temperature  to  tlie  dew 
point.  Then  vapor  must  condense,  and  a  veil  of  fog 
forms.  In  the  early  morning  this  mist  may  often  be  seen 
spreading  over  a  swamp  or  a  stream  bottom.  The  fog 
particle  is  a  minute  drop  of  water,  so  small  that  one  may 
sometimes  walk  in  a  fog  without  becoming  sensibly  wet, 
and  so  small  also,  that  the  particles  do  not  settle  to  the 
ground  by  their  own  weight.  The  centre  of  the  fog  par- 
ticle is  often,  if  not  always,  a  speck  of  dust,  and  it  is 
believed  that  one  of  the  main  causes  for  the  abundant  fogs 
of  London  is  the  presence  of  innumerable  dust  particles 
furnished  from  that  great  cit}^,  and  thus  available  for  the 
condensation  of  vapor  to  form  the  fog. 

There  a^e  various  ways  in  which  fogs  may  be  produced, 
aside  from  that  mentioned  above.  When  one  breathes 
into  the  air  of  a  frosty  morning,  he  forms  a  tiny  fog, 
because  the  warm,  vapor-laden  breath  has  its  temperature 
reduced  by  the  cold  air,  until  the  dew  point  is  reached. 
On  a  very  large  scale  nature  is  making  fogs  of  a  similar 
kind.  In  the  Atlantic,  along  the  path  of  the  European 
steamers,  near  Newfoundland,  extensive  fog  banks  abound. 
Here  there  are  two  currents  of  water,  one  cold  and  moving 
southward  from  the  Arctic,  the  other  warm  and  flowing 
northward  from  the  Tiopics.  The  former  is  the  cold  Lab- 
rador current,  the  latter  the  Gulf  Stream.  When  winds 
from  the  south  pass  over  the  warm  Gulf  Stream,  and  after 
becoming  warm  and  humid  pass  on  over  the  cold  Labrador 
current,  they  are  often  chilled  until  their  temperature  is 
reduced  to  the  dew  point,  when  a  fog  is  produced.  Some- 
times a  similar  fog  is  caused  on  the  land,  when  a  warm, 


134 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


humid  wind  from  the  south  passes  northward  over  the  cold 
land,  in  autumn,  spring,  or  sometimes  even  in  winter. 
During  sucli  conditions,  however,  the  fog  may  not  extend 
over  all  the  land,  but  occurs  only  over  low,  swampy  places 
or  lakes. 

On  the  other  hand,  a  cold  wind  blowing  over  the  warm, 
humid  earth  may  cause  the  dew  point  to  be  reached  in 

the  layers  that 
are  near  the  sur- 
face. Some  of 
the  fogs  of  the 
Gulf  Stream 
region  have  the 
same  origin  as 
this,  when  cold 
air  from  the  Lab- 
rador current 
flows  over  the 
Gulf  Stream, 
chilling  the 
warm,  humid  air  that  exists  there.  By  one  of  these  sev- 
eral causes  fogs  may  fill  valleys,  so  that  from  the  enclosing 
hills  or  mountains  one  may  look  down  upon  a  great  sea 
of  fog  (Fig.  61),  through  which  perchance  the  church 
steeples  rise,  while  all  else  is  hidden  from  sight.  The 
fogs  of  the  land  soon  disappear  before  the  warm  sun, 
which  eats  them  up  by  evaporation,  as  if  by  magic ;  but 
the  heavy  fog  banks  of  the  ocean,  or  of  the  Arctic,  may 
remain  for  days,  until  a  change  in  the  weather  causes 
them  to  disappear. 

Haze.  —  Oftentimes,  particularly  in  summer  and  autumn,  the  air  is 
blue  and  hazy,  so  that  distant  landscapes  are  softened  by  a  veil  of 


Upper  surface  of  a  sea  of  fog.    Looking  down  into  a 
valley  from  a  mountain. 


MOISTURE  IN    THE  ATMOSPHERE  135 

haze  which  otherwise  might  not  be  detected.  Sometimes  the  haze  so 
increases  that  distant  objects  are  obscured.  This  phenomenon  is 
generally  due  to  the  presence  of  an  unusual  number  of  dust  particles ; 
and  after  dry  spells,  when  forest  fires  have  been  extensive,  and  rains 
have  not  come  to  remove  the  dust,  the  air  may  become  exceedingly 
hazy.  In  addition  to  these  causes,  it  seems  probable  that  some  haze 
results  from  a  form  of  liquefied  vapor,  in  which  the  particles  are  even 
less  numerous  and  more  minute  than  in  the  lightest  of  fogs. 

Mist.  —  There  are  times  when  the  air  is  filled  with  a  mist  of  par- 
ticles larger  than  those  of  fog,  and  yet  smaller  than  the  usual  rain- 
drops. This  mist  may  be  due  either  to  the  growth  of  the  fog  particles, 
until  they  become  so  large  that  they  settle  to  the  earth,  or  else  to  the 
combination  of  various  such  particles,  until  the  same  result  is  produced. 
The  latter  may  happen  when  wind  is  blowing  the  fog  about,  so  that 
the  movement  will  cause  numerous  collisions  of  fog  particles,  until 
they  grow  so  in  size  that  they  must  sink  toward  the  earth.  Then  as 
they  settle,  they  strike  other  particles,  and  so  increase  in  size  still 
more. 

Clouds :  Cloud  Materials.  —  A  cloud  may  be  formed  of 
smoke,  or  of  steam  issuing  from  a  locomotive;  but  in 
nature  nearly  all  clouds  are  caused  by  the  natural  conden- 
sation of  vapor  in  the  air,  when  the  temperature  reaches 
the  dew  point.  Therefore  we  may  expect  that  clouds  will 
be  composed  either  of  fog,  mist,  rain,  snow,  or  ice  parti- 
cles. Balloonists  and  travellers  among  high  mountains 
prove  that  this  is  actually  the  case,  for  in  such  journeys, 
clouds  are  entered  and  even  passed  through.  A  fog  or 
mist  may  be  truly  said  to  be  nothing  more  than  cloud 
resting  on  the  earth.  In  climbing  a  mountain  one  may 
see  a  cloud  above  him,  he  may  enter  it,  perhaps  finding 
it  to  be  only  fog,  and  passing  above  it,  and  looking  down 
upon  its  upper  surface,  he  may  see  the  same  appearance  as 
that  caused  by  a  veil  of  fog  in  a  small  valley.  Indeed, 
during  a  rain  storm,  when  the  clouds  rest  upon  the  hill- 


136 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


sides,  one  may  easily  ascend  into  them  and  see  exactly  of 
what  they  are  made. 

Forms  of  Clouds,  —  There   is  nothing  in  nature  more 
beautiful  than  the  forms  and  colors  assumed  by  clouds. 

Being  com- 
monly about 
us,  we  perhaps 
do  not  give 
them  as  much 
attention  as 
they  deserve. 
They  dot  the 
sky  in  patches 
or  clusters, 
having  every 
variety  of 
form,  and  con- 
stantly vary- 
ing in  outline. 
It  would  seem 
an  almost  im- 
possible task 
to  classify 
and  name  all  clouds,  and  so  indeed  it  would  be,  if  we 
tried  to  find  a  name  for  every  variety  of  form.  How- 
ever, there  are  certain  types  which  are  fairly  easy  to 
recognize. 

.  Sometimes  the  sky  is  nearly  or  quite  overcast  by  clouds, 
massed  into  layers  or  bands,  giving  them  a  stratified 
appearance.  These  are  called  stratus^  and  they  generally 
lie  low  in  the  heavens,  perhaps  with  their  bases  resting 
against  the  distant  hillside.     It  is  this  class  that  accom- 


Clouds  upon  a  cliff  in  the  Yosemite. 


MOISTURE  IN   THE  ATMOSPHERE 


137 


panies  the  cyclonic  storms  described  in  the  last  chapter. 
Another  type  is  that  which  comes  on  a  hot  summer  day, 
and  is  commonly  called  the  "thunder  head"  (Fig.  56). 
The  name  for  this  is  the  cumulus  (Fig.  63),  and  it  consists 
of  a  bank  of  cloud  particles  rising  from  a  nearly  level  base, 
whose  elevation  is  several  thousand  feet  above  the  surface. 
Above  this,  domes  of  cloud  masses  rise  several  thousand 


Fig.  63. 
Cumulus  clouds. 


feet  higher.  These  are  among  the  most  beautiful  of  the 
clouds,  and  when  seen  in  the  east  after  a  summer  thunder 
storm,  especially  when  lighted  and  colored  by  the  rays  of 
the  setting  sun,  they  furnish  a  spectacle  which  may  well 
arouse  our  admiration.  From  both  stratus  and  cumulus 
clouds,  rain  may  fall,  and  the  rain-producing  cloud  is 
called  the  nimbus.  Both  stratus  and  cumulus  clouds  are 
generally  so  dense^  that  when  they  pass  before  the  sun,  its 
rays  are  cut  off. 


138 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Oftentimes  there  are  thin  clouds,  which  scarcely  or  only 
partially  intercept  the  sun's  rays.     These  are  high  in  the 

air,  and  measurements  that 
have  been  made,  show  that 
they  are  often  five  or  six 
miles  from  the  earth.  Some- 
times they  are  so  thin  and 
veil-like  that  the  stars  shine 
through  them  at  night.  It  is 
known  that  these  are  made, 
not  of  fog  particles,  but  of 
ice  spicules,  which  are  so 
transparent  that  light  easily 
passes  through.  They  are  so 
high  that  the  liquid  form  of 
water  cannot  exist,  because  of 
the  low  temperatures  which 
occur  there  even  in  summer.  These,  which  are  often 
plumed  and  feathery,  are  called  cirrus  clouds  (Fig.  64), 
and  they  are  the 
highest  of  all. 
It  is  in  them 
that  coronas, 
halos,  etc.,  are 
often  devel- 
oped. 

Between  these 
three  types  there 
is  every  gradation : 
sometimes  the  cir- 
rus are  stratified,  Fig.  65. 
and  they  are  then                            Strato-cumulus  clouds, 


Fig.  04. 
Cirrus  clouds. 


MOISTURE  IN  THE  ATMOSPHERE 


139 


Fig.  66. 
Cirro-cumulus  clouds. 


called  cirro-stratus;  at  times  the  feathery  form  is  replaced  by  little 
banks,  resembling  small  cumulus  clouds  high  in  the  air,  and  these 
are  called  cirro-cumulus  (Fig.  66),  or  if  they  are  more  like  cumu- 
lus than  cirrus, 
cumulo-cirrus.  By 
combining  the 
three  names  into 
similar  compound 
words,  names  may 
be  formed  for 
most  of  the  com- 
mon clouds  of  the 
sky  (Fig.  65). 

Causes  of 
Clouds.  —  Gener- 
ally the  cause  of 
clouds  is  the  same 

as  that  of  other  visible  forms  of  water  in  the  air,  —  the  condensation 
of  vapor.  The  most  common  way  in  which  vapor  is  condensed  in  the 
air,  is  by  lowering  the  temperature  to  the  dew  point.  This  may 
be  caused  by  convection,  and  the  cumulus  clouds  are  commonly  formed 
by  this  means.  The  air  near  the  surface  rises  upon  being  warmed, 
and  as  it  does  so,  cools  (Fig.  18).  Starting  with  a  certain  amount  of 
vapor,  if  the  rising  continues,  this  cooling  must  bring  about  condensa- 
tion whenever  the  proper  temperature  is  reached,  as  it  will  be  at  a 
certain  height,  the  elevation  of  which  will  depend  largely  on  the 
relative  humidity  of  the  air  at  the  beginning.  This  is  why  cumulus 
clouds  have  level  bases,  for  these  represent  the  elevation  at  which 
condensation  began;  and  as  the  air  continues  to  rise,  more  vapor 
condenses  above  this,  forming  the  beautiful  piles  of  cloud  banks. 

Vapor-laden  air,  coming  in  contact  with  a  cool  surface,  may  form 
clouds,  exactly  as  it  may  cause  fog  near  the  ground.  From  this  cause 
clouds  often  gather  around  mountains  and  even  hills.  Or,  again,  as 
in  the  case  of  fog,  air  currents  of  different  temperatures  may  produce 
clouds.  For  instance,  a  cold  layer  of  air  moving  over  a  warm  and 
humid  layer,  may  chill  the  latter  near  the  contact  and  cause  clouds  to 
form.  That  there  are  such  currents  in  the  air,  may  be  inferred  by 
watching  the  clouds,  when  it  may  often  be  seen  that  those  at  different 


140  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

levels  are  moving  in  two  or  more  directions.  Probably  many  of  the 
clouds  of  the  upper  air  are  caused  in  this  way,  and  it  now  seems  cer- 
tain that  this  is  one  of  the  causes  for  the  dense  layers  of  stratus  clouds 
in  cyclonic  storms. 

Rain.  — If  as  a  result  of  any  one  of  the  causes  mentioned 
above,  the  condensation  of  the  vapor  forms  particles  large 
enough  to  fall  through  the  air,  rain  is  formed.^  Often- 
times such  drops  start  from  the  cloud  and  fail  to  reach  the 
ground,  being  evaporated  on  the  way,  because  not  enough 
drops  are  produced  to  satisfy  the  dryer  layers  of  air 
through  which  they  are  passing.  We  may  often  see 
stieamlets  of  such  rain  descending  from  the  summer 
clouds  and  gradually  dying  out  in  the  air. 

A  fog  particle  may  grow  to  the  size  of  a  raindrop  by 
condensation  of  vapor  around  it,  so  that  its  size  constantly 
increases ;  and  then,  starting  to  fall  from  the  cloud,  other 
particles  are  added  to  the  drop  by  collision,  until  perhaps 
the  raindrop  has  grown  to  large  size.  There  is  every 
gradation  from  these  down  to  the  tiny  fog  particles. 

Clouds  furnish  the  birthplace  for  most  raindrops,  and 
their  production  is  merely  a  continuation  of  the  process 
which  makes  the  cloud ;  but  a  cloud  may  be  formed  with- 
out going  far  enough  to  cause  rain,  as  we  all  may  see  from 
the  fact  that  rain  fails  to  fall  from  most  of  them.  When 
the  process  of  raising  air  by  convection,  or  chilling  it  by 
contact  with  colder  bodies,  either  of  air,  water,  or  land,  has 
gone  far  enough,  rain  must  fall,  and  this  is  particularly 
liable  to  happen  when  warm  humid  air  is  present,  for  then 
there  is  much  vapor  to  condense.  This  is  the  case  during 
the  hot  days  which  prevail  in  the  humid  tropical  belt,  and 
in  our  own  country  when  thunder  storms  develop  in  the 
1  Provided  the  temperature  is  above  the  freezing  point. 


MOISTURE  IN  THE  ATMOSPHERE  141 

hot  summer  afternoons.  The  rising  air  in  hurricanes, 
the  air  currents  in  cyclonic  storms,  and  the  damp  air  of 
the  ocean  blowing  against  rising  land,  along  the  margins 
of  continents,  al^o  bring  about  conditions  favoring  the 
formation  of  rain. 

Hail.  —  Rarely,  in  summer,  when  thunder  storms  are  present,  balls 
of  ice  fall  to  the  ground,  sometimes  of  such  size  and  with  such  force 
as  to  break  windows  and  cause  nmch  damage  to  vegetation.  These 
are  really  ice  balls  made  of  layers  of  clear  and  cloudy  ice,  and  they 
represent  the  freezing  of  water  high  in  the  air,  where  the  temperature 
is  low.     Little  is  known  of  the  mode  of  formation  of  these  remarkable 


IBP 

^i^^^p^i^^i 

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^pmi 

W 

^^m^     \^B 

^ 

r^S 

jJPRW' 

J^^^Hs.*  %  ,           ffnHp 

1 

^^^iij^i^i^iiiigii^i 

P 

iP*t-i'--i''- 

1 

^^^^^H 

r      , 

^^^^^H 

tf 

^v    ^  ^^^B^H 

.  j^^^^^^M 

li 

Wfy               Jism\ 

Fig.  67. 

Photograph  of  large  hailstones.    A  ruler,  marked  in  Inches,  shows  the  size  of 

the  hail. 

hailstones,  but  they  are  an  unusual  result  of  vapor  condensation,  and 
are  apparently  formed  when  the  air  is  in  violent  commotion,  and  cer- 
tainly at  an  elevation  where  the  temperature  is  low.  They  differ  from 
snow  in  not  being  made  of  feathery  crystals  caused  by  the  solidifica- 
tion of  vapor. 

Snow.  —  During  a  winter  storm,  when  the  temperature 
is  near  the  freezing  point,  a  heavy,  damp  snow  may  fall, 
and  reaching  the  ground,  cover  it  with  slush.  Later,  by 
a  slight  rise  in  temperature,  the  snow  may  change  to  rain. 


142  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

although  there  has  been  no  difference  in  the  appearance 
of  the  clouds.  After  the  storm  clears,  we  may  see  that, 
while  rain  has  been  falling  on  the  low  ground,  the  neigh- 


/ 


JKT^  ^-^ 


-7^-^r*t 


'  \ 


Fig.  68. 
Photograph  of  actual  snow  flakes. 

boring  highlands  have  been  whitened  with  snow.  During 
such  a  condition  we  may  leave  the  rain,  and  climbing  a 
high  hill,  ascend  into  the  region  where  snow  is  falling, 
passing  first  through  the  zone  where  rain  and  snow  fall 
together. 

Or  the  storm  may  start  as  rain,  and  gradually,  as  the 
thermometer  falls,  change  to  snow,  until  finally  the  deli- 
cate  crystals    or  snow  flakes  are   dry,  and   as  they  fall 


MOISTURE  IN   THE  ATMOSPHERE 


143 


VertietO.  SeeticTU 


RAIN  GAUGE 


accumulate  on  the  ground.  These  snowflakes  are  true 
crystals  of  feathery  and  beautiful  form,  and  they  are  the 
result  of  the  operation  of  a  law  in  nature  whose  products 
are  well  known  to  us,  but  whose  cause  is  not  understood. 
Thip  law  is,  that  upon  solidification  from  the  liquid  or 
vaporous  condi- 
tion, many  sub- 
stances will  take 
on  definite  geo-  l 
metrical  forms,  as  J 
quartz  and  other  j 
crystals  do,  or  as  ! 
salt  may  be  made 
to  do  by  allowing 
a  solution  of  salt 
in  water  to  evap- 
orate. 

In  the  case  of 
snow,  the  vapor  is 
condensed   at   a 


Fig.  69. 


iuner  cylinder; 
the  funnel. 


temperature  below 

freeyino- noint  and  Rain  gauge.    5,  outer  cylinder;  C,  ii 
ireezing  point,  anu       ^^  ^  ^^^^  ^^^^^  left-hand  figures), 

hence  one  at  which 
water  cannot  be  produced,  so  that  as  vapor  is  given  out, 
it  takes  the  solid  form  directly.  So  the  snow  crystal 
gradually  grows,  following  the  definite  laws  of  crystal 
growth,  until  the  beautiful  snow  flake  is  formed  by  con- 
stant additions  of  vapor.  There  is  a  great  variety  of  form 
in  these  flakes,  but  they  all  follow  the  same  law  of  geo- 
metrical perfection.  Snow  crystals  are  not  frozen  rain, 
for  this  would  form  balls  of  ice  or  sleet;  but  they  are  truly 
the  result  of  crystallization  of  water  vapor. 


144  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Measurement  of  Rainfall.  — The  instrument  for  measuring  rain- 
fall is  called  the  rain  gauge.  This  is  a  cylinder  of  metal  which  stands 
in  another  cylinder  whose  area  is  10  times  as  great ;  and  upon  it  is 
a  funnel  whose  top  has  the  same  area  as  the  larger  cylinder.  The 
rain  falls  upon  the  funnel  in  the  same  amount  as  it  would  on  the 
ground,  and  running  down  the  sides,  escapes  through  a  small  orifice 
which  opens  into  the  small  inner  cylinder,  where  the  water  collects  in 
the  bottom.  Since  the  area  of  this  inner  cylinder  is  ^^  that  of  the  area 
of  the  collecting  funnel,  the  depth  of  the  water  is  10  times  as  great 
as  it  would  be  if  gathered  in  a  cylinder  with  the  same  area  as  the 
funnel.  The  object  of  this  increase  is  to  make  it  possible  to  measure 
even  small  rains,  for  by  this  exaggeration  of  depth,  an  inch  of  rain 
becomes  10  inches  deep.  The  measurement  of  the  depth  of  the 
rain  is  made  by  means  of  a  stick  graduated  in  inches  and  tenths  of 
inches.  By  an  inch  of  rain  it  is  meant  that  had  the  rain  sta3'ed  where 
it  fell,  it  would  have  formed  a  water  layer  one  inch  deep.  By  various 
means  the  rainfall  may  be  automatically  recorded. 

For  measuring  the  snowfall,  the  rain  gauge  may  be  used,  the  snow 
being  melted,  and  then  the  water  measured  with  a  stick,  as  before ;  or 
the  depth  of  the  snow  upon  a  level  tract  may  be  measured.  Since  it 
is  customary  to  report  snowfall  in  its  equivalent  amount  of  rain,  it  is 
necessary  to  convert  this  measurement  of  snow  depth  into  rainfall. 
No  perfect  rule  can  be  given  for  this  change,  because  the  amount  of 
rain  represented  in  a  fall  of  snow,  varies  with  its  dryness  or  dampness. 
There  is  more  water  in  a  given  depth  of  damp  snow  than  in  an  equal 
depth  when  it  is  dry.  However,  in  ordinary  snow  a  deptli  of  about 
10  inches  is  equal  to  one  inch  of  rain. 

Nature  of  Rainfall.  — There  is  much  difference  in  rain. 
Sometimes  the  drops  are  tiny,  ahiiost  like  fog  particles, 
while  at  other  times  they  are  large.  In  some  cases,  espe- 
cially when  the  air  is  very  humid,  as  in  summer,  the  drops 
are  large  and  very  numerous,  so  that  in  a  short  time,  per- 
haps in  a  quarter  of  an  hour,  an  inch  of  rain  falls,  while 
in  other  cases,  when  the  drops  are  small  and  not  near 
together,  the  rainfall  of  several  days  may  not  make  an 
inch.     There  is  also  much  difference  in  the  snow,  some. 


MOISTURE  IN   THE  ATMOSPHERE  145 

especially  in  midwinter,  being  very  dry  and  feathery,  while 
in  other  cases,  when  the  temperature  of  the  air  is  nearly 
down  to  freezing  point,  the  snow  crystals  are  damp  and 
matted  together. 

Distribution  of  Rain.  —  Asa  general  statement,  it  may 
be  said  that  there  is  a  decrease  in  the  amount  of  rainfall 
from  the  warm  tropical  belt  toward  the  poles  (Plate  12). 
This  would  be  expected,  because  the  warm  air  of  the 
equatorial  regions  carries  much  vapor,  while  that  of  the 
polar  zone  has  little  to  give.  Hence  a  slight  change  in 
the  temperature  of  the  former  place  will  cause  more  vapor 
to  be  condensed  than  a  great  change  in  the  colder  latitudes. 
Also  there  is  generally  a  heavier  rainfall  on  the  ocean, ^ 
and  near  the  coast,  than  in  the  interior  of  continents. 
This  again  is  easily  understood,  for  the  air  over  the 
water  has  more  vapor  than  that  far  from  the  sea.  In  the 
United  States  this  is  very  well  shown,  for  the  rainfall 
decreases  from  the  Atlantic  and  Gulf  coasts,  westward  and 
northward,  until  near  the  base  of  the  Rocky  Mountains 
there  is  not  enough  rain  for  purposes  of  agriculture.  The 
same  difference  is  shown  in  Russia,  the  steppes  of  south- 
eastern Russia  being  very  much  dryer  than  the  climate 
of  western  Europe. 

This  difference  between  seashore  and  interior  is  even 
better  illustrated  when  the  air  that  goes  inland  is  obliged 
to  pass  over  mountains  on  its  way,  as  is  the  case  in  the 
desert  and  semi-desert  region  of  the  Great  Basin,  between 

1  The  rainfall  on  the  ocean  is  known  to  be  heavy,  although  few  meas- 
urements have  been  made  there.  Vessels  move  about  from  place  to  place, 
and  it  is  only  upon  the  islands  that  measurements  of  rain  can  be  kept  for 
any  length  of  time.  Hence  the  rainfall  chart  does  not  include  the 
precipitation  over  the  ocean. 

L 


146  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

the  Sierra  Nevada  and  Rocky  Mountains.  Here  the  air, 
flowing  up  the  mountain  sides,  is  cooled,  and  hence  caused 
to  give  up  much  of  its  vapor,  so  that  when  it  descends 
on  the  other  side,  and  passes  over  the  plateau,  it  is  dry. 
This  is  one  of  the  two  most  important  causes  for  deserts, 
and  it  explains  the  Great  American  Desert  of  Arizona. 

Since  winds  ^vhich  blow  over  mountains  lose  their  vapor 
as  the  air  rises,  the  sides  of  the  mountain  against  which 
the  wind  blows  are  well  watered,  and  particularly  if  the 
air  comes  from  the  sea.  This  is  shown  in  desert  countries, 
like  our  Great  Basin,  where  mountains  rise  above  the  arid 
plateau.  So  little  rain  falls  upon  the  lower  ground  that 
trees  cannot  exist,  and  other  forms  of  vegetation  grow 
only  scantily  (Fig.  70) ;  but  in  the  mountains  rains  are 
abundant,  and  forests  exist.  Even  in  a  distance  of  a 
few  miles  there  may  be  a  great  difference  in  rainfall. 

In  the  United  States  this  cause  accounts  for  the  heavy 
rain  in  the  state  of  Washington,  w^here  the  prevailing 
westerlies  blow  from  the  warm  Pacific  water  against  the 
sides  of  the  mountains.  It  is  also  shown  on  the  world 
chart,  wherever  the  trade  winds  blow  fi'om  the  ocean 
against  the  rising  continents,  as  in  Central  America  and 
the  coast  of  South  America  (compare  Plates  8  and  9 
with  12). 

The  best  illustration  of  the  effect  of  mountains  that  can 
be  found  in  the  world  is  in  India,  where  the  warm,  humid 
summer  monsoon  wind  blows  from  the  Indian  Ocean 
against  the  mountains.  Here  is  found  the  heaviest  rain- 
fall in  the  world.  In  most  of  eastern  United  States  the 
rainfall  varies  from  30  to  60  inches,  but  there  it  is  493 
inches.  The  heaviest  rainfall  in  this  region  occurs  during 
the  months  of  June,  July,  and  August,  and  sometimes 


,    MOISTURE  IN   THE  ATMOSPHERE  147 

more  rain  falls  in  a  single  day  than  we  have  in  a  great 
part  of  the  eastern  United  States  in  an  entire  year.  So 
heavy  is  the  fall  of  rain  that  all  the  soil  is  washed  from 
the  rocks  of  some  of  the  steep  mountain  sides. 

When  the  trade  winds  blow  over  the  land  toward  a 
coast,  the  air  is  coming  down  grade  and  becoming  warm; 
and  hence,  instead  of  yielding  vapor,  they  have  their 
power  to  take  it  increased.  Then  they  are  drying  winds, 
and  as  a  result  deserts  are  often  produced.  This  is  the 
reason  why  some  west  coasts  in  the  trade-wind  belt  are 
arid,  as  in  the  case  of  western  South  America.  The 
desert  of  Sahara  is  explained  in  a  similar  way.  Here 
the  trades,  after  rising  over  the  highlands  of  northern 
Africa,  both  descend  and  go  to  the  southwards,  thus  hav- 
ing a  double  cause  for  warming,  and  hence  for  being 
drying  winds. 

Where  the  air  rises  in  the  belt  of  calms,  it  cools  and 
gives  up  vapor,  forming  the  copious  rains  of  that  belt, 
which  is  one  of  the  rainiest  in  the  world.  As  the  belt  of 
calms  migrates  northward  and  southward  with  the  seasons 
(Plates  8  and  9),  this  belt  of  heavy  rains  changes  position, 
and  in  this  way  the  southern  edge  of  the  Sahara  region, 
though  dry  in  one  season,  is  well  watered  in  the  oppo- 
site. 

The  rain  of  temperate  latitudes  is  partly  due  to  similar 
convectional  movement,  as  illustrated  in  our  thunder 
storms,  partly  to  air  coming  from  the  ocean,  and  moving 
both  up  grade  and  northward  toward  colder  regions,  and 
partly  to  the  great  cyclonic  storms,  which  pass  over  the 
country,  and  which  in  reality  furnish  us  with  most  of 
our  rainfall  (Chapter  VIII).  In  the  belt  of  prevailing 
westerlies,  west  coasts  are  more  humid  than  east  coasts, 


148  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

because  the  general  direction  of  the  air  is  then  from  ocean 
to  land. 

Distribution  of  Snowfall.  —  Over  a  great  part  of  the  earth  snow 
never  falls,  though  everywhere  the  clouds  of  the  upper  air  are  formed 
in  a  zone  of  perpetual  cold,  where  vapor  never  condenses  in  any  other 
than  the  solid  form.  Therefore  if  a  mountain  peak  reaches  high 
enough,  even  at  the  Equator  the  temperature  may  be  sufficiently  low 
to  cause  snow  in  place  of  rain.  In  the  temperate  latitudes,  freezing 
temperatures  are  often  found  on  the  high  mountains,  even  during  the 
summer,  so  that  in  such  places  rain  rarely  falls.  Upon  these  moun 
tains,  such  as  the  Alps,  there  are  great  snow  fields,  from  which 
glaciers  may  extend  down  into  the  valleys  (Chapter  XVII). 

Over  the  greater  part  of  the  temperate  lands,  some  snow  falls  every 
winter;  but  it  is  only  in  the  higher  latitudes  that  much  accumulates 
on  the  ground.  Even  though  situated  on  the  same  parallel,  less  snow 
falls  in  the  dry  interior  lands  than  near  the  coast,  where  the  air, 
though  warmer,  contains  more  vapor  to  furnish  snow.  Nevertheless 
there  is  less  snow  exactly  on  the  coast  than  at  a  short  distance  inland, 
because  though  there  is  more  vapor,  the  temperature  is  often  so  high 
that  rain  falls,  while  snow  comes  over  the  cooler  inland.  This  is 
very  well  shown  on  the  New  England  coast,  where  the  snowfall  is 
much  less  at  Nantucket  and  Cape  Cod  than  in  central  Massachusetts ; 
and  in  New  York,  where  less  falls  in  New  York  city  than  at  Buffalo. 

Within  the  Arctic  circle  most  of  the  precipitation  is  in  the  form 
of  snow,  though  in  summer,  even  as  far  as  explorers  have  gone,  some 
rain  falls  over  the  sea  and  on  the  land  near  sea  level.  On  the  higher 
ground  of  these  latitudes,  snow  falls  both  in  summer  and  winter,  and 
so  throughout  the  year  this  portion  of  the  earth  is  wrapped  in  snow 
and  ice,  and  great  glaciers  cover  most  of  the  land  (Chapter  XVII). 


CHAPTER   X 

CLIMATE 

Meaning  of  the  Word  Climate. — Every  day  there  are 
changes  in  temperature,  and  perhaps  also  in  wind  direc- 
tion, abundance  of  clouds,  dryness  or  dampness  of  the  air, 
etc.  These  changes  fi-om  day  to  day  are  called  changes  in 
the  iveather.  Climate  includes  a*nd  averages  these  weather 
conditions.  Thus  we  say  that  some  places  have  a  dry  cli- 
mate, others  humid,  some  variable,  others  equable.  A 
variable  climate  is  one  in  which  the  weather  changes 
frequently,  while  in  an  equable  climate  there  is  little 
weather  change.  Therefore,  to  consider  climate  we  must 
look  somewhat  at  the  question  of  weather.  The  elements 
of  the  weather  are  wind,  rain,  clouds,  sunsliine,  tempera- 
ture changes,  etc. ;  and  the  elements  of  climate  are  the 
same,  excepting  that  less  attention  is  paid  to  the  single 
cases,  and  more  to  the  general  result. 

Climatic  Zones.  —  One  might  divide  the  earth  into  cli- 
matic zones  on  the  basis  of  rainfall,  or  any  of  several  features 
of  air  change;  but  it  is  most  common  to  make  temperature 
the  basis.  A  perfectly  natural  division  may  be  made 
according  to  the  altitude  of  the  sun  in  tlie  heavens,  and 
therefore  according  to  the  latitude  of  the  place.  This 
gives  us  the  tropical,  temperate,  and  frigid  zones,  five  in 
all;  but  in  anyone  of  these  there  is  so  much  difference 

U9 


150  FIRST  BOOK  OF  PHYSICAL   GEOGRAPnT 

from  place  to  place,  that  in  each  of  the  zones  there  are 
many  different  climates.  For  instance,  there  is  a  great 
difference  between  the  climates  of  Florida  and  Boston, 
and  between  each  of  these  and  that  of  San  Francisco,  or 
St.  Louis,  or  Helena,  Montana,  though  all  of  these  are  in 
a  single  great  zone,  the  north  temperate. 

There  are  many  variations  in  climate,  and  to  discuss 
this  subject  fully  would  require  several  volumes  of  this 
size.  However,  some  idea  of  the  general  climatic  features 
of  the  world  may  be  gained,  if  we  take  only  a  few  places 
as  types.  To  do  this  properly,  it  will  be  necessary  to 
further  subdivide  each  of  the  zones  into  oceanic  (or  sea- 
coast  and  insular),  interior,  and  mountainous.  Then  also 
there  are  desert  climates ;  and  within  the  tropics  there  are 
differences  between  the  climate  of  the  trade-wind  belt  and 
that  of  the  doldrums. 

There  is  very  much  less  difference  in  the  tropical  and 
frigid  zones  than  in  the  temperate.  In  the  latter  there  is 
an  almost  interminable  variety,  and  while  in  each  of  the 
former  zones  here  are  also  differences  in  climate,  these 
are  less,  because  the  tropical  zone  is  prevailingly  warm,  and 
the  frigid,  cold,  while  the  places  in  the  temperate  zone 
may  be  now  warm  and  now  cold.  Moreover,  those  por- 
tions of  the  temperate  latitudes  which  are  near  the  tropics, 
have  much  higher  temperatures  and  less  variation  than 
those  near  the  frigid  zones.  In  the  consideration  of  the 
climates  of  the  world,  we  will  first  take  the  warm  equa- 
torial belt,  then  the  frigid  zone,  and  then  the  temperate. 

Climates  of  the  Tropical  Zone :  Belt  of  Calms.  —  This  is 
the  zone  where  the  midday  sun  stands  nearly  vertical  at 
all  times  of  the  year,  and  it  is  therefore  the  warmest  belt. 
Here  it  is  that  the  air  is  rising,  as  the  trade  winds  blow 


CLIMATE  151 

in  from  either  side.  Calms  prevail  here,  because  the  air 
ceases  to  move  horizontally,  and  ascends.  In  this  belt 
sailing  vessels  may  linger  for  days,  and  even  weeks,  with- 
out having  a  wind  of  sufficient  force  to  drive  them  through 
to  the  zone  of  the  trade  winds.  On  the  ocean,  where  the 
doldrums  are  best  developed,  the  air  both  of  day  and  night 
is  warm  and  humid  throughout  the  year.  The  heat  of  the 
daytime,  although  great  and  oppressive,  is  tempered  some- 
what by  the  ocean,  and  this  is  the  most  equable  climate 
in  the  world.  Because  the  air  is  rising,  and  hence  cool- 
ing, there  is  heavy  rain  throughout  the  year;  and  during 
the  day,  when  the  sun  shines,  and  the  air  rises  more  per- 
ceptibly, clouds  form  and  copious  rain  falls. 

Over  the  land  there  is  less  rain  and  the  temperature 
is  less  equable.  Because  of  radiation  from  the  land, 
the  daytime  is  hot  and  the  nights  cool.  Because  of  the 
absence  of  ocean  water  to  supply  vapor,  the  air  is  less 
humid,  and  hence  the  rainfall  is  less.  Also  the  air  is  less 
calm,  for  over  various  parts  of  the  land  there  are  differ- 
ences in  temperature,  which  cause  breezes  to  arise.  Never- 
theless, even  over  the  land,  this  is  a  rainy,  relatively  calm, 
and  very  warm  belt.  In  it  are  situated  the  heavily  forested 
rainy  districts  of  central  Africa  and  tlie  Amazon  valley. 
These  change  gradually  to  less  dense  forests  on  either 
side,  and  in  Africa  gradually  give  place  to  the  dry  desert 
of  Sahara,  on  which  almost  no  vegetation  can  grow. 

As  the  belt  of  calms  migrates  with  the  seasons,  the  position  of  the 
heavy  rains  changes,  so  that  on  the  margin  of  the  tropical  rain  belt, 
there  is  a  climate  which  is  dry  in  one  season  and  very  wet  in  the 
other.  This  is  shown  in  South  America,  where  the  llanos  of  Vene- 
zuela are  rainy  during  the  summer  season,  and  dry  in  the  winter,  and 
also  in  the  campos  of  Brazil,  south  of  the  Equator,  which  are  well 
watered  in  winter  and  dry  in  summer. 


152  FIRST  BOOK  OF  PHYSICAL   GEOGRAPnV 

The  Trade-Wind  Belt.  —  There  is  much  more  variety  in 
the  climate  of  this  zone  than  in  the  belt  of  calms.  Being 
within  the  tropics,  the  temperature  is  everywhere  high 
excepting  on  the  lofty  mountain  tops,  where  the  climate 
is  almost  frigid.  Among  these  mountains  one  may  jour- 
ney from  the  zone  of  perpetual  summer,  to  that  where  the 
conditions  of  spring  prevail  throughout  the  year,  and 
then,  going  still  higher,  one  may  rise  above  the  elevation 
where  timber  can  grow,  and  finally  into  a  region  where 
the  nights  are  cold  and  the  days  cool,  and  perhaps  even 
as  cold  as  those  of  the  northern  winter. 

Although  in  respect  to  temperature  there  is  a  resemblance  between 
these  climates  of  high  altitudes  and  those  of  the  frigid  zone,  there  is 
this  difference,  that  even  though  the  weather  is  cold,  the  sun  rises 
liigh  in  the  heavens  in  midday  at  all  times  of  the  year. 

Over  the  ocean  the  trade  winds  blow  with  wonderful 
steadiness, moving  constantly,and  with  distinct  strength, in 
one  direction.  They  are  warm  and  carry  much  vapor,  taken 
as  they  pass  over  the  sea;  but  since  they  are  blowing  from 
colder  to  warmer  latitudes,  their  temperature  is  constantly 
rising,  and  hence  they  are  able  to  take  much  more  vapor. 
Therefore  as  they  blow  over  the  sea,  they  do  much  work  of 
evaporation,  and  here  they  are  not  especially  rainy  winds ; 
but  that  the  trade  winds  are  carrying  much  vapor  is  proved 
by  the  fact  that  when  the  air  has  its  temperature  lowered, 
when  rising  in  the  belt  of  calms,  it  precipitates  quantities 
of  moisture. 

When  blowing  over  the  land,  the  trade  winds  may  pro- 
duce deserts,  as  they  do  in  the  Sahara  north  of  the  Equator, 
and  in  Australia  and  South  Africa  south  of  the  Equator. 
The  desert  climate  is  one  of  extreme  heat  in  the  da3^  fol- 
lowed by  a  cool,  or  in  winter  really  cold,  night  (Fig.  25). 


CLIMATE 


153 


Because  of  the  dryness  of  the  air,  which  permits  heat  to 
readily  reach  the  ground  throughout  the  daytime,  and 
allows  it  to  be  radiated  at  night  with  almost  equal  ease, 
the  temperature  range  of  the  desert  is  great.  As  the  name 
indicates,  a  desert  climate  is  one  of  great  dryness,  rarely 


Fig.  70. 
The  desert  vegetation  in  the  far  west. 

if  ever  a  climate  in  which  no  rain  falls  (Plate  12),  but  one 
in  which  there  is  little  precipitation,  and  this  only  in  cer- 
tain seasons.  In  the  desert  there  is  not  enough  rainfall 
to  support  any  but  the  most  hardy  forms  of  desert  plants. 
On  the  land,  the  direction  of  the  trade  winds  is  often 
changed  as  one  part  of  the  land  becomes  warmer  than 
another,  causing  a  circulation  as  the  air  attempts  to 
equalize  the  differences  in  pressure  thus  produced.     In 


154  FIEST  BOOK  OF  PHYSICAL   GEOGRAPnT 

this  way  the  trade  winds  are  sometimes  deflected  from  their 
course,  and  caused  to  move  toward  the  heated  land,  as  in 
the  case  of  western  Africa  (Plates  8  and  9),  and  also  on 
manj^  oceanic  islands,  where  the  sea  breeze  blows  and  air  is 
drawn  in  from  the  ocean.  Then  the  climate  of  the  trade- 
wind  belt  may  be  changed  from  one  of  moderate  dryness 
to  one  of  heavy  rainfall  (Plate  12) ;  for  as  the  air  blows 
in  over  the  heated  land,  either  passing  up  the  grade  of 
the  land,  or  rising  by  convection,  the  dew  point  is  soon 
reached,  clouds  are  formed,  and  rain  falls.  The  trade- 
wind  belt  is  also  rainy  on  many  east-facing  coasts,  for 
as  the  air  blows  against  the  land,  being  forced  to  rise,  it 
gives  up  some  of  its  vapor,  causing  heavy  rains,  as  in  the 
case  of  South  America  both  to  the  north  and  south  of  the 
Equator  (Plate  12). 

The  Indian  Climate.  —  A  peculiar  climate  is  found  on  the  plains  of 
India,  within  the  trade-wind  belt,  where  the  air  movement  is  modified 
by  the  monsoon  effect  of  Asia.  Here  there  are  three  seasons,  the  hot 
summer,  the  rains,  and  the  winter.  The  hot  summer  begins  in  April 
and  lasts  until  June,  and  during  this  time  the  air  is  hot  and  dry,  and 
the  temperature  of  the  day  reaches  above  100°  in  the  shade,  and 
sometimes  110°  or  115°.  Everything  becomes  dry,  and  it  is  almost 
impossible  for  an  Englishman  to  take  exercise,  excepting  at  night  and 
just  before  the  dawn.  Everything  withers  before  the  scorching  west 
winds  which  blow  from  the  sandy  wastes  of  the  Indus  valley. 

Then  in  June  there  comes  a  calm,  in  which  the  heat,  still  intense, 
becomes  even  more  suffocating,  because  there  is  no  movement  of  the 
air;  and  every  one  prays  for  the  south  and  east  winds  of  the  summer 
monsoon,  which  bring  rain  and  some  relief  from  the  steady  heat. 
Finally  clouds  appear,  and  rain  falls,  which  during  this  season,  last- 
ing over  a  month  or  two,  is  of  daily  occurrence.  Under  the  influence 
of  the  heavy  rainfall,  plants  flower  a  second  time,  having  previously 
been  in  leaf  and  flower  in  February  or  March.  The  intense  dryness 
is  followed  by  equally  intense  dampness,  and  then,  toward  the  close 
of  the  rains,  the  climate  is  once  more  almost  unendurable. 


CLIMATE  155 

By  the  beginning  of  October  the  winter  monsoon  begins,  and  from 
then  until  December  the  cool  air,  blowing  from  the  highlands  of 
northern  and  central  India,  transforms  the  hot  plains  to  a  region  with 
a  deliciously  cool  climate,  in  which  the  air  is  clear  and  dry.  Then 
the  weather  becomes  so  cold  that  fires  are  needed  in  the  evening, 
during  the  months  of  December  and  January.  In  February  the  warm 
weather  begins,  and  a  sort  of  spring  visits  the  land,  inducing  vegeta- 
tion to  break  forth ;  and  this  is  then  followed  by  the  hot,  dry  season, 
which  discourages  the  thriving  vegetation,  and  causes  it  to  wither 
until  it  again  bursts  forth  in  the  tj^ue  growing  season  of  wet  weather. 

Climates  of  the  Frigid  Zones  :  The  South  Frigid  Zone.  — 
Very  little  is  known  about  the  climates  of  the  south  frigid 
belt,  but  there  seem  to  be  two  different  climates,  that  of 
the  ocean,  and  that  of  the  high,  ice-covered  land  of  the 
Antarctic  continent,  which  appears  to  entirely  enwrap  the 
South  Pole.  Probably  these  climates  are  very  similar  to 
those  of  the  Arctic  Ocean,  on  the  one  hand,  and  the  ice- 
covered  land  of  Greenland  on  the  other;  but  they  have 
not  been  studied. 

JSfear  the  Arctic  Circle.  —  Within  the  Arctic  there  is  a 
progressive  increase  in  the  severity  of  the  climate  as  one 
proceeds  northward.  In  the  extreme  southern  portion, 
near  the  sea  level,  the  summer  sun  reaches  about  as  high 
in  the  heavens  as  it  does  in  northern  United  States  during 
the  late  autumn.  At  night-time  it  drops  down  to  the 
northern  horizon,  and  at  midnight  the  earth  is  lighted 
either  by  the  dim  sunlight  or  bright  twilight.  Therefore, 
although  the  sun  is  low  in  the  heavens,  it  shines  most  or  all 
of  the  day,  and  the  summer  weather  is  cool,  but  not  cold. 
The  storms  of  temperate  latitudes  affect  this  region,  and 
hence  there  is  much  variety  of  weather,  with  alternately 
cool  and  clear  conditions,  followed  by  a  cloudy  sky,  per- 
haps with  rainy  weather,  similar  to  the  changes  in  the 


156 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


United  States.  The  temperature  is  so  high  that  rain  falls 
instead  of  snow,  which  is  the  form  of  precipitation  during 
most  of  the  year. 

In  the  winter  the  sun  rises  high  enough  to  cause  twi- 
light at  midday,  and  the  night  is  therefore  constant. 
Then  there  is  a  season  of  prevailing  cold.  The  storms 
cause  cold  or  warm  winds,  and  clear  weather  or  snow,  and 
these  changes  are  determined,  not  by  the  direct  influence 
of  the  sun,  but  by  outside  causes.  There  is  little  or  no 
daily  rise  and  fall  of  temperature,  but  the  variations  are 

mainly  gov- 
erned by  move- 
ments of  the 
air  caused  by 
the  cyclon- 
ic storms  of 
the  temperate 
zone.  Between 
this  season  of 
winter  cold 
and  summer 
coolness,  there 
are  two  seasons  when  the  sun  rises  and  sets ;  and  then,  in 
addition  to  the  causes  mentioned,  the  temperature  changes 
from  day  to  night.  There  are  therefore  four  different 
seasons,  with  quite  distinct  characteristics. 

In  the  Higher  Latitudes.  —  Further  north  these  climates 
become  even  still  different.  The  winter  night  is  marked 
by  intense  cold,  with  temperatures  generally  ranging  be- 
tween 21°  and  60°  (and  probably  more)  below  zero,  though 
now  and  then  rising  above  freezing  point,  when  a  warm 
wind  is  caused  by  some  movement  of  the  air.     During 


The  midnight  sun,  northern  Norway. 


CLIMATE 


157 


this  season  the  land  is  deeply  snow-covered,  and  the  sea 
coated  with  ice.  As  the  darkness  of  the  winter  night  is 
replaced  by  the  spring-time,  soon  the  midday  becomes 
comfortably  warm,  while  the  nights  are  cold.  Then  the 
snow  begins  to  melt,  and  the  sea  ice  to  break  up  and  float 
away  to  the  south  (Fig.  72),  where  it  eventually  melts  in 
the  warmer  waters  of  the  more  temperate  latitudes. 


Fkj.  72. 

The  ice-covered  sea  off  Cumberland  Sound,  Baffin  Land,  summer  of  1896. 

Steamer  Hope  in  the  ice. 

After  this  comes  the  summer,  when  plants  burst  forth 
into  blossom,  and  the  hum  of  insects  is  heard,  being 
warmed  into  life  under  the  influence  of  the  summer  sun, 
tliat  shines  by  night  as  well  as  by  day.  Though  low  in 
tlie  heavens,  the  sun  warms  the  earth,  the  frost  disappears 
from  the  surface,  and  even  at  midnight  the  temperature 


158  FIEST  BOOK  OF  PHYSICAL  GEOGBAPHT 

does  not  descend  to  the  freezing  point.  During  this 
season  rain  may  fall  even  as  far  north  as  man  has  gone. 
After  this  comes  the  autumn,  when  the  darkness  of  night 
again  appears,  and  the  earth,  warmed  slightly  by  day, 
cools  at  night,  so  that  the  bays  begin  to  freeze,  snow  com- 


FiG.  73. 
A  part  of  the  high  coast  of  Greenland,  summer  of  189G.    Latitude  74°  15'. 

mences  to  fall,  and  gradually  the  day  becomes  shorter, 
until  finally  the  winter  night  sets  in,  and  for  weeks,  and 
further  north  even  for  months,  the  sun  is  not  seen. 

The  same  changes  of  sun  occur  in  tlie  high  interior  lands;  but 
here,  because  of  the  greater  elevation,  the  temperature  is  so  low  that 
even  in  summer  there  is  never  rain,  and  never  warmth  enough  to  melt 
the  snow.      Here  perpetual  winter  prevails,  and  the  land  is  deeply 


CLIMATE 


159 


covered  with  snow  and  ice,  as  in  the  case  of  the  whole  interior  of 
Greenland,  and  probably  also  of  the  great  Antarctic  continent.  This 
climate,  which  is  bitter  cold  in  summer,  nmst  become  intensely  severe 
in  winter ;  but  no  one  has  ever  lived  there  to  tell  us  how  low  the  tem- 
peratures descend.   We  do  know  that  the  snowfall  is  extremely  heavy. 


Fig.  74. 

The  Greenland  ice  sheet  showing  a  part  of  the  Cornell  Glacier.  Latitude  74°  15'. 
Taken  from  an  elevation  of  1400  feet.  Fjord  in  foreground  3  miles  wide,  and 
icebergs  in  it,  in  some  cases,  75  feet  high.  The  glacier  covers  all  land  except- 
ing the  island  in  the  ice  (nunatak),  which  is  9  miles  distant. 

There  are  other  differences  in  the  frigid  climate,  the 
most  noteworthy  of  which  is  that  caused  by  ocean  currents. 
The  coast  of  Greenland  is  distinctly  warmer  than  that  of 
Baffin  Land  in  the  same  latitude,  because  a  warm  current 
of  water,  coming  from  the  south,  bathes  the  former  shores, 
while  a  cold  current,  flowing  southward,  passes  Baffin 
Land  and  carries  to  that  coast  the  chill  of  the  ice-laden 
waters  of  the  north. 


160 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Climates  of  the  Temperate  Zone  :  Various  Types.  — Near 
the  frigid  zones  the  temperate  climate  is  quite  like  that 
near  the  Arctic  and  Antarctic  circles ;  and  near  the  tropics 
tlie  conditions  resemble  those  described  for  the  trade-wind 
belt.  Between  these  two  extremes  are  the  great  temperate 
belts  of  variable  climates,  in  which  so  much  of  the  civil- 
ized world  lies.     These  two  zones,  one  south  and  one 

north  of  the  Equator, 
are  characterized  by  pre- 
vailing moderate  tem- 
perature, changing  from 
warm  and  perhaps  hot 
in  summer,  to  cool  or 
cold  conditions  in  win- 
ter. Everywhere  there 
is  generally  a  range  of 
temperature  from  day  to 
night,  and  this  differs  in 
winter  and  summer,  and 
also  in  the  two  interme- 
diate seasons  of  autumn 
and  spring.  These  daily 
temperature  changes  are 
liable  to  be  interrupted 
by  the  cyclonic  and  anticyclonic  disturbances  which  affect 
the  temperate  latitudes.  The  wind  and  weather  changes 
are  influenced  by  the  prevailing  westerlies,  and  mainly 
caused  either  by  differences  in  heat  effect,  or  by  the  low- 
and  high-pressure  areas,  which  pass  from  west  to  east  over 
the  higher  latitudes  of  the  belt  (Chapter  VIII). 

The  climates  and  weather  of  the  south  temperate  zone  resemble 
those  of  the  northern,  excepting  in  the  fact  that  there  is  less  u-regu- 


FiG.  75. 

A  cold  wave,  March  13,  1888.  Temperature 
iiulicated  by  shading.   Isobars  also  shown. 


CLIMATE 


161 


larity,  because  the  southern  hemisphere  is  occupied  mainly  by  water. 
This  allows  less  range  of  temperature,  and  less  irregularity  of  winds, 
which  prevailingly  blow  from  the  west.  Near  the  tropics,  in  both 
hemispheres,  the  climate  of  the  temperate  zones  is  modified  by  the 
conditions  of  the  horse  latitudes,  where  the  air  is  settling,  the  winds 
light  and  variable,  with  frequent  calms,  and  the  sky  generally  clear. 
In  this  belt  of  settling  air  there  are  some  deserts,  as  for  instance  on 
the  east  coast  of  southern  South  America. 


90  ^^«j  W.it^*' JO     0      ^\1i 


Fig.  76. 
Map  showing  snowfall  of  United  States  in  inches.     All  in  temperate  belt. 


United  States  Climates. — In  northern  United  States, 
southern  Canada,  and  Europe,  we  find  the  characteristic 
weather  conditions  and  variable  climates  of  the  temperate 
latitudes.  This  variability  has  been  sufficiently  described 
for  the  United  States  in  the  chapter  on  Storms,  and  what 
is  said  there  applies  to  Canada  and  Europe,  and  in  gen- 
eral to  other  parts  of  the  middle  temperate  belt.  By  the 
passage  of  cyclonic  storms  and  anticyclones,  the  daily 
range  of  temperature  in  winter  may  be  replaced  by  warm 
spells,  or  by  cold  weather,  and  in  summer  by  hot  or  by 
cool  spells.     A  cold  wave,  overspreading  the  land,  may 


162  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

cause  the  temperature  to  drop  even  at  midday,  and  cover 
the  northern  United  States  with  a  blanket  of  cold  air, 
producing  temperatures  varying  from  freezing  to  40° 
below  zero  (Fig.  75).  Or  on  the  contrary,  warm  winds 
from  the  south,  moving  toward  a  storm  centre,  may  cause 
the  temperature  to  rise,  even  at  night,  and  to  become  so 
high  that  a  thaw  occurs,  causing  the  snow  to  melt  from 
the  ground  (Figs.  53  and  54). 

As  these  changes  occur,  the  direction  of  the  wind  varies, 
being  now  from  the  south,  now  from  the  west,  north,  or 
east;  and  now  the  sky  is  clouded,  and  perhaps  rain  or 
snow  is  falling,  and  then  the  sky  is  clear,  and  the  vault 
of  the  heavens  a  beautiful  blue.  Day  by  day,  and  week 
by  week,  these  changes  occur,  and  no  one  can  tell  what 
the  weather  will  be  from  week  to  week,  excepting  to  know 
that  it  will  be  variable,  day  after  day.  This  is  in  striking 
contrast  to  the  ever  dry  climate  of  a  desert,  the  constant 
winter  cold  of  parts  of  the  Arctic,  the  uniform  humidity 
of  the  doldrum  belt,  or  the  permanent  winds  of  the  lands 
influenced  by  the  trades. 

Difference  bettveen  United  States  and  Europe.  —  Within  this  belt 
of  variable  temperate  climate  there  is  much  variety  from  place  to 
place.  The  great  city  of  St.  Petersburg  is  situated  in  nearly  the  same 
latitude  as  southern  Greenland  and  northern  Labrador,  —  places  in- 
habited only  by  Esquimaux  and  a  few  Europeans  who  live  there  for 
purposes  of  trade ;  Berlin  and  London  lie  in  the  latitude  of  southern 
Labrador,  a  sparsely  settled  region,  having  a  climate  of  almost  Arctic 
rigor ;  and  New  York  lies  in  the  latitude  of  southern  Italy  and  Greece, 
places  with  warm  and  almost  subtropical  climates. 

There  are  two  reasons  for  these  conditions,  one  the  fact  that  the 
prevailing  westerlies  blow  over  eastern  America,  after  having  passed 
across  the  land,  while  those  of  Europe  have  blown  across  the  ocean 
water.  The  second  reason  is  that  the  water  of  the  eastern  Atlantic, 
on  the  European  coast,  is  warmed  by  an  ocean  current  from  the  south 


CLIMATE  163 

(the  Gulf  Stream),  while  the  American  shores  are  bathed  by  a  frigid 
current  (the  Labrador)  from  the  icy  Arctic  sea  (Chap.  XTII) .  For  simi- 
lar reasons  the  western  coast  of  the  United  States  is  warmer  than  the 
eastern,  and  also  warmer  than  the  eastern  coast  of  Asia  in  the  same 
latitude ;  but  the  difference  between  the  climates  of  the  Asiatic  and 
American  coasts  is  less  than  that  just  described,  because  no  cold  Arctic 
current  bathes  the  shores  of  eastern  Asia. 

Variation  ivith  Altitude.  —  There  are  also  noticeable  dif- 
ferences in  the  climate  of  the  temperate  zone  according  to 
altitude.  In  a  small  way  this  may  be  seen  in  any  mountain- 
ous district,  like  that  of  New  England,  where  the  valleys 
are  very  much  warmer  than  the  mountain  tops  (Fig.  31). 
For  instance,  Mt.  Washington  in  New  Hampshire  is  en- 
wrapped in  cold  air  even  in  summer,  while  the  lowlands 
to  the  east,  in  New  Hampshire  and  Maine,  are  covered 
with  a  blanket  of  hot  air ;  and  by  autumn  the  top  of  this 
mountain  is  covered  with  snow,  while  in  winter  the  tem- 
peratures are  exceedingly  low  and  the  snowfall  heavy.  In 
the  same  way  the  Alps,  which  lie  in  the  latitude  of  south- 
ern France,  where  the  summer  is  hot  and  the  winter  not 
extreme,  are  so  cold  that  snow  falls  upon  their  summits 
even  in  summer,  so  that  there  are  great  fields  of  perpetual 
snow  and  glaciers  among  the  mountains  (Chap.  XVII). 

Differences  between  Ocean  and  Land.  —  Again  there  is  a 
variation  in  climate  from  the  sea  to  the  interior.  The  cli- 
mate of  New  York  city  is  warmer  and  more  equable  than 
that  of  Ohio,  and  this  in  turn  is  less  extreme  than  that 
of  Wyoming,  all  of  which  are  in  the  same  latitude.  The 
climate  of  Boston  is  less  severe  than  that  of  the  interior 
of  Massachusetts,  and  from  place  to  place  along  the  coast 
one  finds  many  variations  (Figs.  24  and  30). 

At  Cape  Ann,  Mass.,  a  point  nearly  surrounded  by  the  equaDle 
ocean,  the  temperature  during  the  cold  midwinter  does  not  descend 


164  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

so  far  as  at  places  10  miles  inland;  and  in  spring,  being  surrounded 
by  the  cold  ocean  water,  vegetation  does  not  so  quickly  develop  as  it 
does  at  Cambridge,  which  lies  but  a  short  distance  inland.  The 
leaves  begin  to  appear  upon  the  maple  trees  in  Cambridge  fully  a 
week  earlier  than  at  Cape  Ann.  Throughout  the  fall,  the  ocean  water, 
warmed  during  the  summer,  prevents  radiation  on  the  land  from 
cooling  the  air  over  this  cape  to  such  low  temperatures  as  those 
reached  a  short  distance  away ;  and  hence  some  frosts,  which  occur 
a  few  miles  from  the  shore,  do  not  visit  the  cape. 

These  same  differences  in  climate  are  shown  near  lakes, 
as  for  instance  along  the  shores  of  Lake  Erie,  in  western 
New  York.  Here  the  conditions  are  so  equable,  near  the 
lake  shore,  that  an  extensive  grape-raising  industry  lias 
developed,  while  upon  the  hillsides,  two  or  three  miles 
away,  this  industry  is  not  possible. 

On  an  even  more  notable  scale  the  influence  of  the 
ocean  upon  climate  may  be  illustrated  by  contrasting  the 
Bermuda  Islands  with  central  Georgia  on  the  same  paral- 
lel of  latitude.  The  former,  surrounded  entirely  by  warm 
ocean  waters,  does  not  have  extremely  high  temperatures 
in  summer,  while  in  winter  the  nights  are  never  cold  and 
rarely  cool,  and  frosts  are  practically  unknown.  The 
vegetation  has  a  tropical  aspect  (Chap.  XI),  and  the  Ber- 
mudas in  this  respect  resemble  the  Bahamas  and  Florida, 
which  lie  much  further  south.  In  central  Georgia,  the 
sun,  having  the  same  altitude,  warms  the  land  in  summer, 
causing  hot  days,  while  in  winter,  although  the  climate 
is  not  extreme,  frosts  are  by  no  means  uncommon,  and 
the  nights  are  often  cold.  Thus  in  the  temperate  lati- 
tude there  is  an  infinite  variety  in  the  weather;  and 
equally  great  is  the  variation  in  the  climates  of  different 
parts  of  the  great  zone. 


CHAPTER   XI 

DISTRIBUTION   OF   ANIMALS    AND   PLANTS 

Zones  of  Life.  —  There  are  three  great  zones  of  life 
inhabited  by  different  assemblages  of  animals  and  plants, 
—  the  ocean,  the  land,  and  the  fresh  water.  In  each  of 
these  there  are  subzones  in  which  the  animals  and  plants 
differ  because  of  variations  in  temperature.  For  instance 
there  are  very  different  organisms  in  the  tropical  belt 
from  those  of  the  frigid  or  even  the  temperate  zone,  and 
this  applies  to  ocean  and  fresh-water  as  well  as  to  land 
animals.  Besides  these  there  are  other  zones  in  the  sea 
and  deeper  lakes,  for  the  nature  of  the  flora  and  fauna 
varies  with  depth.  Also  in  the  larger  bodies  of  water  there 
are  changes  with  distance  from  shore,  and  also  along  the 
shore,  as  the  nature  of  the  coast  varies.  There  are  also 
numerous  subdivisions  of  the  zones  of  land  life.  The 
creatures  that  live  among  the  mountains  differ  from  those 
of  the  plain,  while  those  of  the  humid  seacoast  climate 
bear  very  little  resemblance  to  those  of  the  desert.  The 
subject  of  the  distribution  of  animals  and  plants  is  there- 
fore a  very  complex  one,  which  in  a  book  of  this  scope 
can  be  treated  only  in  a  brief  and  most  general  way. 

Life  in  the  Ocean 

Plants. — In  the  sea  both  plants  and  animals  exist  in 
great   abundance,  though   the    latter   greatly  exceed   the 

166 


166 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


former  in  importance.  Excepting  at  the  very  coast  line, 
none  of  the  higher  flowering  plants,  so  common  on  the  land, 
are  found.  Upon  salt  marshes  (Chap.  XVIII),  there  are 
numerous  species  of  plants,  resembling  those  of  the  land. 
On  the  coast  of  Florida  and  other  tropical  lands,  the 
mangrove  tree  is  able  to  live  with  its  roots  in  salt  water ; 

and  in  protected  places  along 
these  coasts  there  are  veri- 
table jungles  of  mangrove 
swamps  (Fig.  77),  into  which 
the  salt  water  enters. 

Elsewhere  in  the  sea  the 
plant  life  belongs  to  the  lower 
forms  of  vegetation,  notably 
the  seaweed.  Upon  that  part 
of  the  rocky  coast  of  New 
England  which  is  exposed  at 
low  tide,  these  plants  produce 
a  mat  (Fig.  78),  and  the  sea- 
weed also  grows  upon  the 
bottom,  near  the  coast.  It  is 
limited  to  shallow  water,  be- 
cause at  depths  of  a  few  hun- 
dred feet  not  enough  sunlight  passes  through  the  ocean 
water  to  perform  the  work  necessary  for  plant  growth. 
Therefore,  the  great  expanse  of  ocean  bottom,  where  the 
water  is  deep,  is  devoid  of  vegetable  life,  being  in  this 
respect  a  great  desert  extending  over  nearly  three-fourths 
of  the  earth's  surface.  Seaweed  needs  to  have  a  solid  base 
on  which  to  grow,  and  hence,  on  the  exposed,  sandy  shores, 
where  the  waves  keep  the  sand  particles  in  constant  move- 
ment, these  delicate  organisms  cannot  exist. 


Fig.  77. 
A  mangrove  swamp,  Bermuda. 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS       167 

There  is  also  much  seaweed  floating  about  on  the  surface  of  the 
ocean,  especially  in  the  warm  waters  of  the  tropical  zone  in  the  mid- 
ocean  ;  and  sometimes  this  gathers  over  such  great  areas,  that  sailing 
vessels  have  their  progress  retarded  in  passing  through  them.  These 
are  called  "grassy"  or  Sargasso  seas,  like  that  which  lies  between 
Spain  and  the  West  Indies,  in  which  the  species  of  Sargassum  are 
most  abundant  (Plate  16). 


Animals.  —  The  abundance  of  animal  life  in  the  sea  is 
marvellous,  and  there  is  no  part  of  its  surface  or  bed  which 
is  not  inhabited.  We  may 
recognize  three  great  zones  of 
animal  life  in  the  ocean : 
(1)  The  littoral,  or  that  of 
the  seacoast;  (2)  the  abi/  sal, 
or  that  of  the  ocean  botto  n ; 
and  (3)  the  pelagic,  or  that 
of  the  surface. 

Faunas  of  the  Coast  Line 
(^Littoral  faunas) .  —  The  sea- 
coast  faunas  vary  greatly  from 
place  to  place.  Some  creatures 
swim  in  the  surf,  some  cling 
to  the  seaweed,  many  attach 
themselves  firmly  to  the  rocks, 
others  burrow  in  the  mud, 
and  in  all  of  these  places 
many  move  about  from  place 

.        T  -,■,  .  -,.  Seaweed  mat.    Shore  of  Cape  Ann, 

to  place,  walking  or   Crawhng         Mass.    Exposed  between  tides. 

over  the   bottom.     Hence  as 

we  pass  along  the  coast,  going  from  a  rocky  headland  to 
a  sandy  beach,  and  then  to  a  muddy  fiat  in  an  enclosed 
bay,   we    find   three    entirely   different   types    of   animal 


Fig.  78. 


168  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

colonies,   even  though  the    distance   be  no  more   than  a 
mile. 

There  are  also  great  differences  in  the  animal  life  ac- 
cording to  the  temperature  of  the  water.  Within  the 
Arctic  almost  no  organisms  live  on  the  rocky  shores, 
because  in  winter  these  are  ice-bound,  and  as  the  tide  rises, 
it  grinds  against  the  shore  with  such  power  that  no  life 
can  withstand  its  effects.  But  below  the  reach  of  the  ice, 
many  animals  and  seaweeds  exist. 

These,  as  well  as  those  of  the  temperate  latitudes,  though  abun- 
dant, are  much  less  so  than  the  myriads  of  creatures  that  dwell  in  the 
warm  waters  of  the  tropics.  Moreover,  they  are  much  less  delicate 
and  beautiful ;  and  in  their  plain  and  rugged  forms  they  tell  of  the 
struggle  against  the  rigors  of  winter,  to  which  the  tropical  animals 
are  not  subjected.  The  latter,  bathed  constantly  in  warm  water,  and 
at  all  times  furnished  with  an  abundance  of  food,  become  marvellously 
beautiful  and  varied  in  form  and  color.  Similar  differences  are 
noticed  on  the  land,  both  in  the  animal  and  vegetable  kingdoms. 

It  is  within  the  waters  warmed  by  the  tropical  sun,  and 
particularly  where  this  water  is  in  circulation  as  warm 
ocean  currents,  that  we  find  the  littoral  faunas  in  all  their 
luxuriance  of  development.  These  currents  not  only  bear 
the  equable  conditions  of  constant  warmth,  but  also  an 
abundance  of  floating  animals,  which  thrive  in  the  warm 
water.  The  dwellers  on  the  seacoast  and  shallow  bottoms, 
usually  anchored  firmly  in  place,  cannot  go  to  seek  their 
food,  but  must  have  it  brought  to  them ;  and  therefore 
warm  currents,  laden  with  animalculse,  furnish  them  with 
an  abundant  food  supply. 

Under  these  favorable  conditions,  reefs  of  coral  develop,  and  one  who 
has  never  seen  a  coral  reef  (Fig.  79),  can  form  no  real  conception  of 
the  vast  numbers  a.n4  wpn^erful  variety  of  the  animal  life  clustered 


DISTRIBUTION  OF  ANIMALS  AND  PLANTS 


169 


together  in  these  colonies.  Drifting  about  in  a  boat,  one  may  gaze 
down  upon  a  bottom  covered  with  corals  of  all  colors  and  forms,  with 
millions  of  mouths  wide  open  and  hungry  for  food  that  is  floating 
past.  The  coral  reefs  are  the  gardens  of  the  sea,  and  I  know  of  no 
better  comparison  than  to  a 
garden  on  the  land,  in  which 
plants  of  various  colors  and 
forms  are  growing  in  rank 
profusion.  Nowhere  else  in 
the  world  are  there  so  many 
individuals  and  species  of 
animals  clustered  together 
in  a  small  space,  as  one  may 
see  in  the  coral  reefs  of  Ber- 
muda, the  Bahamas,  and 
many  of  the  coasts  of  the 
tropics. 

Animals  of  the  Ocean 
Bottom  (^Abyssal  Fauna). 
—  It  was  once  thought 
that  no  animals  could 
exist  on  the  bed  of  the 

sea,  where  the  water  has  a  depth  of  at  least  a  mile,  and 
often  two  or  three  miles,  and  where  the  temperature  is 
always  nearly  at  the  freezing  point,  and  where  a  darkness 
like  that  of  night  constantly  reigns.  Now,  however,  as  a 
result  of  much  study,  we  know  that  this  great  realm  is 
inhabited  by  animals  not  greatly  different  from  those  of 
the  ocean  surface.  Fishes  swim  about,  shellfish  crawl 
over  or  burrow  into  the  mud,  and  shrimp,  sea-anemones, 
and  many  other  kinds  of  ocean  animals  exist  there  in  great 
numbers.  Some  are  blind,  but  others  have  eyes.  Most 
of  the  animals  dwelling  in  this  zone  depend  for  their 
food  supply  upon  the  remains  of  animals  that  lived  near 


Fig.  79. 

A  part  of  the  Great  Barrier  Reef,  Australia, 
showing  profusion  of  coral  life. 


170 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


the  surface  of  the  sea,  and  upon  dying,  settle  to  the 
bottom.  The  abundance  of  animal  life  on  the  ocean  bed 
is  in  large  part  determined  by  the  amount  of  food  that  is 
thus  supplied. 

There  are  fewer  differences  among  the  animals  of  differ- 
ent parts  of  the  ocean  bottom,  than  in  any  other  great  area  ; 


Fig.  80. 
A  deep-sea  fish. 

the  temperature  is  always  low,  there  are  no  changes  with 
season,  and  none  from  day  to  night,  and  the  temperature 
is  about  the  same  at  the  Equator  as  at  the  Arctic  circle, 
being  nearly  everywhere  below  40°,  day  after  day,  and  year 
after  year.  There  is  therefore  little  cause  for  differences. 
Nearly  everywhere,  as  the  depth  of  the  sea  increases,  the 
temperature  becomes  lower  (Fig.  93),  and  this  is  one  great 
cause  for  the  variation  in  the  faunas  of  the  deep  sea.  Some- 
times warm  currents  of  water  bathe  shallow  parts  of  the 
sea  bed,  and  then,  under  the  more  favorable  conditions, 
greater  numbers  and  different  kinds  of  animals  live  in  the 
shallows,  than  in  the  neighboring  deeper  and  colder  water. 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS 


171 


Life  at  the  Surface  (^Pelagic  Faunas}.  — Here,  there  is 
great  variety  and  abundance  of  animal  life.  Not  merely 
do  the  larger  fishes  swim 
about  singly  and  in  great 
"schools"  or  "shoals," 
containing  tens  of  thou- 
sands in  a  single  group, 
but  the  water  teems  with 
life  of  minute  and  even 
microscopic  size.  Count- 
less myriads  of  these 
tiny  creatures  occupy 
the  water  of  all  parts  of 
the  ocean  surface,  from  the 
tropical  zone  to  the  ever 
frozen  waters  of  the  Arc- 
tic. One  might  sail  over 
the  ocean  without  being 
aware  of  their  existence, 
so  small  are  they ;  but  if 
he  will  drag  the  surface 
with  a  net  having  minute 

meshes,  he  may  gather  these  animals,  and  in  a  dish  of  sea 
water  examine  them  at  leisure. 

Now  and  then,  for  some  unknown  reason,  these  tiny  creatures  com- 
bine to  produce  a  phosphorescent  glow  on  the  water  surface,  and  then 
at  night,  a  gleam  of  silvery  light  marks  the  track  of  the  vessel,  or  sil- 
very drops  fall  from  the  oars.  Each  animalcule  is  emitting  his  share 
of  this  strange  light.  The  abundance  of  these  creatures  is  shown  by 
the  fact  that  the  mammoth  whale  obtains  his  living  from  them, 
swimming  through  the  water  with  his  mouth  wide  open,  and  strain- 
ing the  minute  animals  from  the  water  by  means  of  the  fringed 
whalebones.     These  monsters  obtain  food  by  this  means  not  only  in 


^ 

^^ 

I 

MI) 

\ 

|.> 

N^r 

w 

1^ 

■  .  -  .                          --'» 

Fig.  81. 
A  deep-sea  crinoid. 


172  FIRST  BOOK  OF  PHYSICAL   GFOGRAPUY 

the  warm  waters  of  the  tropics,  but  even  in  the  ice-strewn  Arctic 


seas. 


There  is  little  reason  for  difference  in  the  pelagic  fauna, 
excepting  in  places  so  far  apart,  and  so  different,  as  the 
cold  waters  of  the  frigid  zone  and  the  warm  equatorial 
ocean.  Within  the  tropics,  the  waters  are  always  so  warm, 
and  the  conditions  so  uniform,  that  the  surface  animals 
differ  but  little;  they  swim  about  easily  from  place  to 
place,  or  are  driven  here  and  there  before  the  winds  or 
the  currents,  and  hence  are  widely  distributed.  So  also 
in  the  fiigid  zone,  the  constant  cold  which  prevails  in 
these  waters  favors  uniformit}^  of  life.  In  the  temperate 
latitudes,  however,  there  are  somewhat  greater  differences, 
and  hence  greater  variety.  Near  the  coast,  the  water  \s 
cold  in  winter  and  warm  in  summer,  while  further  out  to 
sea,  the  temperature  is  more  equable,  and  generally  higher, 
because  of  the  presence  of  warm  ocean  currents.  There- 
fore there  are  zones  of  life  here. 

In  the  ocean  there  is  therefore  the  greatest  variety  of 
life  conditions  in  the  littoral  or  seashore  zone,  and  least 
in  the  great  expanse  of  the  deep  sea.  In  each  of  these 
zones  there  is  wide  distribution,  partly  because  the  waters 
are  in  nearly  constant  movement,  partly  because  many  of 
the  creatures  can  swim  about, ^  and  partly  because  the 
temperature  variation  is  not  great,  excepting  in  widely 
separated  regions.  In  the  entire  area  of  tliese  three  gi'eat 
zones,  there  is  a  wonderful  variety  and  abundance  of 
animals. 

1  Even  those  which  are  anchored  have  a  free-swimming  stage  early  in 
Ufe  before  settUng  down  to  the  real  condition  of  maturity. 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS        178 


Life  in  Fresh  Water 

Some  of  the  animals  that  dwell  in  fresh  water  come  from  the  air 
and  land,i  but  there  are  many  species  of  plants  and  animals  which 
live  and  die  in  the  fresh  water.  As  the  animals  and  plants  of  the  land 
vary,  so  do  those  of  the  lakes  and  rivers,  for  among  these  there  are 
differences  from  lowland  to  mountain,  and  from  frigid  to  tropica] 
zones.  Many  of  the  groups  of  animals  of  the  sea  inhabit  the  lakes ; 
but  many,  like  corals,  are  never  found  in  fresh  water.  Some  sea  fish 
(such  as  salmon,  alewives,  etc.)  have  a  habit  of  passing  up  rivers  into 
lakes  to  breed  or  "  spawn,"  and  hence  there  is  often  a  difference  between 
the  faunas  of  fresh-water  bodies  near  the  sea  and  those  remote  from 
it ;  for  not  only  do  the  adults  pass  up  stream  to  lay  their  eggs,  and 
then  come  back  again,  but  the  young  remain  in  the  lakes  for  a  season. 
Through  changes  of  land  and  water,  animals  of  the  sea  have  some- 
times been  obliged  to  remain  permanently  in  the  fresh  water,  and 
then  sea  fish  are  found  in  the  lakes  at  all  times.  Probably  many  ot 
the  lake  and  river  fish  have  come  to  inhabit  their  present  homes  in  a 
similar  way. 

In  the  small  lakes,  and  in  rivers,  there  are  only  slight  diffeiences  in 
the  nature  of  the  animal  and  plant  inhabitants  from  on6  part  to 
another.  There  will  perhaps  be  different  creatures  on  the  swampy 
shores  from  those  of  the  rocky  headlands,  and  between  these  and  the 
inhabitants  of  the  marshy  and  sandy  shores;  but  these  variations 
would  be  slight,  because  the  distance  and  the  difference  in  conditions 
are  not  great.  But  in  large  lakes,  like  the  Great  Lakes,  the  fauna  and 
flora  vary  greatly,  and  in  a  way  similar  to  the  conditions  existing  in 
llie  ocean,  although  in  lakes  there  is  no  such  abundant  variety  of 
animal  life  as  in  the  sea.  There  is  here  a  variable  littoral  fauna  and 
flora,  a  pelagic  zone,  and  a  lake-bottom  zone.  In  the  latter,  as  in  the 
sea,  there  is  little  variety,  because  the  temperature  is  always  low,  and 
the  conditions  constant  and  unfavorable  to  life.  Here  also,  as  in  the 
sea,  plant  life  ceases  below  the  depth  where  the  sun's  rays  cease  to  be 
powerful  enough  to  perform  the  work  needed  by  plants.  Although 
the  species  are  not  the  same,  and  notwithstanding  many  minor  dif- 

^  Such  as  the  young  of  the  mosquito  and  dragon-fly  from  the  air,  and 
the  tree-toad  from  the  land. 


174 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


ferences,  there  is  a  general  resemblance  between  the  life  zones  of  large 
lakes  and  the  sea. 

Sometimes  a  fresh-water  lake  has  its  source  cut  off,  as  the  result 
of  a  change  in  climate  from  moist  to  arid,  when  the  water  evapo- 
rates faster  than  the  rain  can  supply  it;  and  then  gradually  a 
salt  lake  results,  as  in  the  case  of  the  Great  Salt  Lake  and  the 
Dead  Sea.     As  the  fresh  water  changes  to  salt,  many  of  the  animals 

perish,  until  final- 
ly only  a  few  re- 
main, so  few,  in 
fact,  that  the  lake 
is  called  a  dead 
sea.  Even  in  the 
Great  Salt  Lake 
there  are  some  mi- 
nute animals  and 
plants.  The  life 
of  the  salt  lakes 
is  different  from 
that  in  any  other 
part  of  the  world, 
the  chief  differ- 
ence being  in  the 
very  limited  num- 
ber of  species 
and  individuals, 
which  contrasts  so  distinctly  with  the  abundance  of  life  in  ocean  and 
fresh  water. 

Life  on  the  Land 

Plants.  —  The  most  characteristic  life  on  the  land  is  the 
plant  life,  and  yet  animals  are  of  very  high  importance. 
Vegetation  clothes  the  surface  almost  everywhere,  furnish- 
ing dwelling-places  for  many  animals  and  supplying  all 
with  food.  Animals  of  the  land  have  not  the  power  of 
taking  food  directly  from  the  earth ;  but  plants  are  able  to 
convert  earthy  materials  into  substance  which  animals  can 


Fig.  82. 
A  view  in  a  semi-tropical  forest  in  Florida. 


DISTRIBUTION  OF  ANIMALS  AND  PLANTS        175 


use;  and  even  those  creatures  which  do  not  take  their 
food  directly  from  plants,  do  so  more  indirectly  from  other 
animals.  Most  plants  live  at  the  surface  of  the  earth  and 
cling  to  it,  firmly  anchored  in  place. 

..  There  is  a  limit  to  the  abundance  of  plants  in  any  given 
place,  but  this  varies  with  the  conditions.  Under  the 
most  favorable  circum- 
sfences,  as  for  instance 
within  the  tropical  belt  of 
heavy  rains,  the  limit  is  only 
that  of  space.  As  many  as 
can  get  together  and  obtain 
food  from  the  earth,  and  the 
necessary  sunlight  for  plant 
growth,  can  exist  in  this 
place;  and  here  are  found 
tropical  jungles  of  forest 
trees  reaching  high  in  the 
air,  and  a  tangle  of  under- 
growth, forming  an  almost 
impassable  mat  of  vegeta- 
tion. 

On  the  other  extreme,  in 
deserts,  even  though  within 
the  tropics,  the  conditions 
of  sunlight  and  warmth  are 

still  present,  but  the  equally  necessary  water  is  absent, 
and  hence  the  surface  is  only  scantily  clothed  with  the 
desert  grass,  cactus,  and  other  kinds  of  vegetation  which 
can  thrive  amidst  such  adverse  conditions  (Figs.  70  and  83). 

Growing  easily,  because  of  the  favorable  conditions,  plants  in  the 
humid  climates  suffer  little  if  attacked  by  animals ;  but  on  the  desert 


Fig.  83. 

In  the  desert  of  Arizona,  showing  giant 
cactus. 


176 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


the  conditions  of  nature  are  so  adverse  that  any  additional  difficulties 
would  be  fatal.  Hence  the  desert  plants  attempt  to  protect  themselves 
from  the  attacks  of  animals,  attaining  this  end  sometimes  by  means  of 
thorns,  as  in  the  case  of  cactus,  and  sometimes  by  developing  sub« 
stances  in  the  sap  which  cause  the  taste  of  the  plant  to  be  disagreeable, 
as  in  the  sage  brush  of  the  desert. 

Equally  adverse  are  the  conditions  on  high  mountain 
tops  (Fig.  84),  where  the  temperature  is  low,  or  in  the 


Fig.  84. 
A  mountain  peak  on  the  crest  of  the  Andes  in  Peru,  above  the  timber  line. 

Arctic,  where  not  only  is  there  cold,  but  also  a  limited 
amount  of  sunshine.  Approaching  either  of  these  re- 
gions, we  find  the  trees  changing  from  the  deciduous  to 
the  evergreens,  the  forests  becoming  less  dense,  and  then 
-the  trees  more  and  more  scattered  and  stunted,  until  finally 
the  timber  line  is  passed  (Fig.  85),  and  the  only  forms  of 
vegetation  are  those  that  cling  to  the  earth.  Within  the 
Arctic  regions  the  willows  and  other  species  of  trees  creep 


DISTRIBUTION  OF  ANIMALS  AND  PLANTS       177 

along  the  ground,  raising  their  leaves  and  flowers  no  higher 
than  is  necessar}^  for  early  in  the  winter  it  is  vitally 
important  to  secure  a  covering  of  snow  which  shall  pro- 
tect them  from  the  intense  cold.  However,  no  matter 
how  far  north  we  may  go,  even  to  the  highest  latitude 
so  far  visited,  grass  and 
flowers  will  be  found  in 
summer,  wherever  soil 
exists  in  a  position  ex- 
posed to  the  sun  (Fig. 
86).  Upon  the  bare 
rocks  are  innumerable 
lichens  and  mosses,  and 
on  the  hillsides,  and  in 
the  valley  bottoms,  are 
patches  covered  with 
flowering  plants,  but 
there  are  never  any  trees. 
With  change  in  lati- 
tude there  are  many  va- 
riations in  vegetation; 
the  tropical  plants  differ 
radically  from   those  of 

the  temperate  zone,  being  much  more  abundant,  varied, 
and  beautiful.  Fruits  abound,  and  Nature  is  very  prod- 
igal in  her  productions. 


_ 

'-^^^^wH^^^ 

^ 

i^^<^^3H 

m^ 

5^^^ 

IM  ^^•['^'•^KSS^^^aBI 

sflF  '''•?~i». 

1 

Fig.  85. 

A  view  near  the  timber  line  in  the  high 
Rockies  of  Colorado. 


Many  of  the  plants  of  the  world  are  cultivated  by  man,  and  even 
where  he  protects  them,  there  are  distinct  belts  in  which  various 
species  grow  best.  For  instance,  coffee,  bananas,  pineapples,  etc.,  can- 
not be  grown  in  northern  United  States ;  and  barley  or  wheat  thrive 
better  in  the  cooler  climates.  But  in  the  cold  Arctic  zone  it  is 
impossible  to  raise  even  the  hardiest  of  crops,  for  the  summer  season 


178 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


is  too  short  for  any  but  the  native  species,  which  are  accustomed  to 
blossom  and  mature  their  seed  in  the  short  summer  of  these  far 
northern  lands. 

Animals.  —  The  animal  life  of  the  land  may  be  said  to 
live  in  three  great  zones,  or  else  to  spend  a  part  of  their 
time  in  two  or  all  three.  There  are  many  that  dwell  in 
the  air,  and  this  includes  many  of  the  insects  and  most  of 

the  birds,  as 
well  as  some 
others  (such 
as  the  bat). 
Others,  and 
the  majority, 
dwell  on  the 
surface  of  the 
earth,  mov- 
ing about  by 
one  means  or 
another ;  for 
on  the  land 
the  habit  of 

remaining  fixed  in  one  place  is  not  common  as  it  is  in  the 
sea.  An  animal  that  stayed  in  one  place  would  have  little 
chance  to  obtain  its  food  supply,  for  the  air  is  unlike  the 
ocean  in  this  respect.  A  third  group  of  animals  occupies 
the  ground,  either  living  in  it  all  of  the  time,  or  spending 
part  of  the  time  on  the  surface.  There  are  also  many  of 
the  land  animals  which  leave  the  dry  land,  either  in  some 
stage  of  their  development,  or  from  time  to  time,  taking 
to  the  sea  or  fresh  water  for  the  purpose  of  obtaining  a  food 
supply.  Among  these  there  are  some  which  spend  more 
time  in  the  sea  than  on  the  land. 


Fig.  86. 

A  view  in  North  Greenland,  showing  plants  and  flowers 
(Arctic  poppies)  peeping  above  the  snow. 


DISTRIBUTION  OF  ANIMALS  AND  PLANTS       179 

Among  land  animals  there  is  much  less  widespread 
distribution  than  among  the  animals  of  the  sea,  or  the 
plants  of  land  and  sea.  Some,  like  birds  or  insects,  move 
so  readily  that  they  occupy  large  areas.  For  instance, 
ducks  and  geese  which  nest  in  the  Arctic,  spend  the  win- 
ter in  southern  United  States  and  Mexico;  but  most  of 
the  land  animals  move  about  so  slowly,  and  with  such 
difficulty,  that  they  are  restricted  to  relatively  limited 
localities.  Some  species  are  confined  to  certain  small 
tracts. ^ 

As  in  the  case  of  plants,  so  among  the  animals,  there 
is  a  very  great  difference  between  those  of  the  tropics  and 
those  of  the  colder  zones.  The  fauna  of  the  humid  belt 
of  calms,  rivals  the  flora  in  abundance  and  variety.  The 
tropical  forest  is  alive  with  creatures  of  all  classes,  because 
food  is  furnished  prodigally  by  the  abundant  growth  of 
vegetation.  Where  this  is  more  nearly  absent,  as  in  des- 
erts, animals  become  scarce. 

Upon  our  western  plains  the  limited  number  of  insects  and  birds, 
contrasts  strikingly  with  the  abundance  of  these  creatures  in  the 
swamps  of  Arkansas  in  the  same  latitude;  and  the  reptiles  and 
higher  animals  show  an  even  greater  diminution.  The  antelope,  the 
prairie  dog,  a  few  burrowing  animals,  one  or  two  species  of  rabbit, 
the  coyote,  and  a  few  other  species,  which  constitute  the  higher  ani- 
mal life  of  the  desert,  furnish  a  striking  contrast  to  the  scores  of 
mammals  which  exist  in  more  favorable  localities. 

In  ascending  a  mountain  a  similar  change  is  seen,  and 
in  a  journey  to  the  Arctic,  one  finds  the  decrease  in  abun- 

1  For  instance,  the  Australian  land,  surrounded  by  water,  though  not 
far  from  the  large  islands  to  the  northward,  has  animals  so  different  from 
those  of  other  countries,  that  it  may  be  said  to  have  a  fauna  of  its  own. 
Nowhere  else  in  the  world  are  the  kangaroo,  and  the  many  other  strange 
animals  of  Australia,  at  present  living. 


180  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

dance  of  land  animal  life  parallel  to  that  of  the  plant  life. 
For  instance,  in  central  eastern  Greenland,  with  the  excep- 
tion of  the  mosquito,  insects  are  not  numerous  or  abundant. 
Reptiles  and  burrowing  animals  are  entirely  absent,  be- 
cause the  frost  is  in  the  ground  throughout  the  year,  and 
in  winter  the  temperature  is  extremely  low.  Birds,  mainly 
those  which  obtain  food  entirely  from  the  sea,  are  abun- 
dant along  the  coast  in  summer,  but  most  of  these  disap- 
pear with  the  coming  of  winter.  The  chief  land  birds  are 
the  snow  bunting,  ptarmigan,  and  raven.  Reindeer,  foxes, 
and  Arctic  hares  are  the  principal  land  mammals,  and 
these  are  by  no  means  abundant. 

The  polar  bear,  though  coming  to  the  land  now  and  then,  lives 
mainly  on  the  floating  ice  of  the  sea  where  he  obtains  his  food.  The 
seal  and  walrus  crawl  upon  the  rocks,  but  only  now  and  then,  for  they 
spend  most  of  their  time  in  the  water  or  on  the  sea  ice.  This  list  of 
the  land  animal  life  of  this  inhospitable  climate  furnishes  a  wonder- 
ful contrast  to  the  thousands  of  species  that  inhabit  the  tropical  for- 
ests. Moreover  it  is  noticeable  that  the  higher  creatures  of  the  Arctic 
live  there  only  by  protecting  themselves  by  a  thick  covering  of  fur, 
which  keeps  out  the  cold,  and  that  niost  of  them  depend  not  upon  the 
land  for  their  food,  but  upon  the  sea.  In  the  great  ice-covered  wastes 
of  interior  Greenland,  there  is  an  entire  absence  of  both  animal  and 
vegetable  life.    This  is  the  most  absolute  desert  so  far  visited  by  man. 

Distribution  of  Man.  —  Once  was  the  time  when  men 
were  distributed  in  belts,  and  when  races  were  separated 
and  marked  by  distinct  characteristics ;  but  now,  with  the 
progress  of  civilization  and  the  development  of  means  of 
transportation,  the  barriers  have  in  large  measure  been 
removed,  and  before  the  white  race,  the  others  are  dis- 
appearing or  are  being  rapidly  absorbed. 

There  are  still  savage  or  uncivilized  races  which  are 
kept  within  certain  bounds  by  natural  barriers,  as  people 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS        181 

were  in  Europe  several  centuries  ago ;  but  most  races 
have  reached  the  stage  of  development  when  natural 
barriers  are  easily  overcome.  Rivers  no  longer  present 
serious  obstacles  to  travel,  as  they  did  when  the  bounda- 
ries of  some  of  the  European  countries  were  drawn.  Seas 
are  no  longer  crossed  with  difficulty,  as  was  the  case  when 
England,  Scandinavia,  and  other  countries  developed  in- 
dependently of  their  neighbors;  and  mountains  are  no 
longer  such  impenetrable  barriers  as  they  were  in  the  time 
when  it  was  possible  for  a  tiny  state,  like  Switzerland,  to 
exist  independently  in  the  midst  of  greedy  neighbors. 

The  boundary  lines  of  many  countries  were  drawn  in  the  days 
when  even  a  dense  forest  was  a  difficulty  of  a  serious  nature.  For  a 
long  time  the  Appalachian  forests,  aided  to  be  sure  by  the  Indian 
occupants,  served  as  a  barrier  to  the  progress  of  the  American  people, 
and  caused  a  concentration  along  the  Atlantic  coast;  and  it  is  doubt- 
ful whether  the  American  Revolution  would  have  been  successful  had 
it  been  possible  for  the  early  settlers  to  have  spread  themselves  over 
the  western  territory. 

Without  serious  study  one  can  hardly  realize  how  closely 
dependent  upon  geographic  conditions  has  the  develop- 
ment of  the  human  race  been,  although  now  we  have 
nearly  risen  above  this  dependence  on  natural  surround- 
ings. In  his  advance  toward  a  higher  civilization,  man 
has  been  subjected  to  many  of  the  same  influences  that 
have  affected  the  abundance  and  variety  of  plants  and 
animals. 

For  instance,  amid  the  conditions  of  the  tropics,  al- 
though savage,  his  superior  intelligence  and  skill  made 
man  the  master ;  but  his  mastery  cost  so  little  effort,  and 
his  livelihood  was  so  secure,  that  he  did  not  advance  as 
rapidly  as  those  who  were  placed  amid  the  greater  dif- 


182 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Acuities  of  the  temperate  zone.  Here  constant  effort  was 
necessary,  so  that  the  intelligence  and  skill  were  developed 
which  have  made  the  men  of  temperate  latitudes  the  mas- 
ters of  the  world,  and  not  merely  of  other  men,  but  to  some 
extent  of  Nature  herself.     The  people  of  the  Arctic  have 

had  too  severe  a  struggle, 
and  too  little  opportunity, 
and  as  a  result  they  have 
developed  hardly  more  than 
the  races  of  the  tropics ;  yet 
among  uncivilized  peoples, 
one  'will  probably  not  find  a 
more  intelligent  race  than 
the  Esquimaux  of  the  Arctic. 
Modes  of  Distribution  of 
Animals  and  Plants. —  While 
from  place  to  place  there  are 
many  variations  in  the  kind 
of  animals  and  plants,  many 
species  are  widely  distrib- 
uted. The  same  species  in 
some  cases  are  found  in 
Siberia  and  British  North 
America,  or  in  both  eastern  and  central  United  States. 
Those  animals  which  live  in  the  air  or  water  are  most 
easily  distributed,  being  drifted  about  in  these  media. 
One  may  gain  an  excellent  idea  of  the  general  subject  of 
the  modes  by  which  animals  and  plants  are  distributed 
over  the  earth,  by  comparing  the  fauna  and  flora  of  Ber- 
muda with  that  of  the  United  States ;  for  in  the  resem- 
blances and  differences  which  are  found,  the  chief  causes 
for  distribution  are  illustrated. 


Fig.  87. 

A  Bermuda  road  bordered  by  cedar 
groves. 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS       183 

The  Bermudas  lie  about  600  miles  from  the  Carolina  coast,  which 
is  the  nearest  land.  They  form  a  cluster  of  tiny  islands,  absolutely 
alone  in  the  sea,  and  have  never  been  connected  with  North  America; 
but  yet  the  animals  and  plants  are  American  in  kind.  Upon  their 
surface  we  find  the  cedar  (Fig.  87)  and  other  plants  from  the  same 
latitude  in  North  America,  the  cactus  and  Spanish  dagger,  which  on 
the  mainland  exist  on  the  arid  plains,  the  palmetto,  and  other  Bahama 
and  Florida  plants,  the  oleander,  and  scores  of  other  species  common 
in  southern  United  States. 

When  these  islands  were  first  visited,  not  a  single  mammal, 
excepting  the  bat,  was  found  on  the  island.     Insects  of  the  same  kind 


Fig.  88. 
A  bit  of  Bermuda  landscape. 

as  those  of  the  mainland  are  numerous ;  and  birds  are  also  there  in 
considerable  numbers,  particularly  the  ground  dove,  redbird,  blue- 
bird, catbird,  and  a  few  others.  A  tiny  lizard  of  the  same  kind  as 
one  in  the  West  Indies  is  also  found  there. 

How  did  these  come  to  this  remote  island,  and  why  are 
there  no  larger  animals  ?  One  of  the  most  striking  facts 
concerning  the  fauna  of  the  Bermudas,  is  that  the  animal 
life  is  chiefly  made  up  of  species  which  can  fly;  and  every 
year  there  is  proof  that  it  is  this  fact  which  accounts  for 
their  presence.  Robins  and  other  birds  of  passage  stop 
upon  this  land  during  their  annual  migrations,  and  during 
nr  after  heavy  storms,  many  species  of  birds  are  seen  which 


184  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

do  not  naturally  belong  there,  and  which  quickly  disap- 
pear. They  have  been  blown  out  to  sea  by  the  wind,  and 
the  number  which  have  been  thus  driven  away  from  the 
land,  may  be  shown  by  the  fact,  that  although  the  land 
birds  native  to  the  island  number  hardly  more  than  a 
dozen,  185  different  species  have  been  found  there.  Even 
the  tiny  humming-bird  has  been  seen  in  the  Bermudas.  ^ 
No  doubt  for  every  tiny  bird  that  makes  the  journey  in 
safety,  scores  perish  at  sea.  Naturally  then,  the  dwellers 
of  the  air,  either  by  direct  flight,  or  driven  by  accident, 
may  make  a  journey  across  the  sea,  and  better  yet  over 
the  land,  perhaps  starting  a  colony  in  some  new  place  not 
before  occupied  by  this  particular  species. 

Birds,  eating  fruit  upon  the  mainland,  after  arriving 
at  the  island,  or  in  fact  after  making  any  journey,  may 
drop  seeds,  which,  sprouting,  develop  into  plants  that 
start  a  growth  of  a  new  kind  in  this  place.  Also  the 
wind,  carrying  lighter  seeds,  may  drive  them  to  far-distant 
lands.  The  first  lizards  which  came  to  the  Bermudas 
were  no  doubt  carried  there  upon  bits  of  floating  wood, 
moving  in  the  ocean  currents,  which  have  also  carried  the 
shells  now  inhabiting  the  land  and  the  water  which  sur- 
rounds these  islands.  None  of  the  larger  and  higher 
species  of  animals  can  take  such  a  journey;  for  there 
would  be  nothing  to  float  them,  and  in  any  event  they 
probably  could  not  survive  it. 

1  It  may  appear  strange  that  so  small  a  bird  can  make  so  great  a  jour- 
ney ;  but  it  must  be  remembered  that  there  are  many  logs  and  bits  of  wood 
floating  in  the  sea,  and  that  these  will  serve  as  a  resting-place  for  the 
birds  that  are  forced  to  make  this  flight  against  their  will.  It  is  by  no 
means  uncommon,  when  sailing  far  out  of  sight  of  land,  to  see  some  small 
land  bird  wearily  flying  toward  the  ship,  where  he  rests  for  awhile  ni 
the  rigging,  before  taking  up  his  flight  in  the  effort  to  again  reach  the  land 
from  which  he  has  been  driven. 


DISTRIBUTION   OF  ANIMALS  AND  PLANTS        185 

These  are  the  means  by  which  animals  and  plants  are 
distributed:  some  make  direct  journeys,  some  come  by 
chance,  and  some  are  carried  by  accident.  On  the  land 
they  move  slowly  along,  spreading  out  into  whatever  new 
territory  they  may  be  able  to  occupy. 

This  process  of  extension  of  species  into  areas  previously  unoccupied 
by  them,  is  well  illustrated  in  the  case  of  many  of  the  weeds,  such  as 
the  field  daisy,  introduced  by  chance  into  this  country  during  the 
Revolution,  and  now  one  of  the  commonest  of  plants ;  or  of  the 
Canadian  thistle,  which  has  extended  its  range  so  as  to  become  a  pest 
in  farming  districts.  Many  insects,  like  the  Colorado  beetle  or  potato 
bug,  and  other  animals,  like  the  English  sparrow,  the  latter  a  European 
species,  finding  themselves  in  a  new  region  favorable  to  their  develop- 
ment, have  multiplied  and  spread  in  a  wonderful  manner.  In  Australia 
the  rabbit,  introduced  from  Europe  not  many  years  ago,  has  become 
a  national  pest. 

Man  has  now  come  upon  the  scene,  and  has  become  the 
most  potent  of  all  agents  in  the  distribution  of  animals 
and  plants.  Formerly,  by  their  own  or  by  accidental 
movements,  organisms  had  spread  about  over  the  land,  so 
that  the  world  was  divided  into  fairly  definite  zones,  each 
species  having  found  a  place  for  itself  and  occupying  a 
restricted  area;  but  man  is  interrupting  all  this,  killing 
off  species  here,  and  introducing  them  there,  so  that  very 
often  it  is  difficult  to  say  which  species  are  native  and 
which  introduced.  For  instance,  in  Bermuda,  many 
plants  carried  there  by  man  have  taken  such  a  footing  on 
the  islands  that  in  some  cases  it  is  almost  impossible 
to  say  that  they  were  not  there  before  man  came. 

Barriers  to  the  Spread  of  Life.  —  The  most  effective  bar- 
rier to  the  spread  of  life  is  temperature.  No  matter  by 
what  means  the  cocoanut  or  the  coffee  berry  were  carried 
to  northern  United  States,  they  could  not  grow;  nor  would 


186  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

one  of  our  pines  or  spruces  find  the  tropical  belt  a  con- 
genial home.  There  is  therefore,  on  land  as  well  as  sea, 
a  limit  to  the  distribution  both  of  animals  and  plants, 
dependent  largely  upon  temperature.  Therefore  the  spe- 
cies of  the  north  temperate  and  Arctic  belts  are  not  at 
all  the  same  as  those  of  the  southern  hemisphere,  even 
where  the  climatic  conditions  are  the  same,  for  they  cannot 
pass  the  great  tropical  harrier. 

For  the  same  reason  mountain  ranges  serve  as  a  partial 
barrier;  for  because  of  the  low  temperatures,  many  species 
cannot  cross  them,  though  some  of  the  more  hardy  species 
are  able  to  make  the  journey,  and  others  pass  around  the 
ends,  so  that  these  elevations  are  not  complete  barriers. 
Organisms  accustomed  to  life  in  a  humid  climate  cannot 
survive  on  a  desert.,  and  therefore  this  also  serves  as  a 
partial  barrier.  Even  a  river^  or  a  chain  of  lakes.,  may 
mark  the  limit  of  spread  of  some  species ;  and  sometimes 
a  forest.,  or  an  open  country^  may  serve  as  a  barrier;  for 
the  forest-dweller  may  not  be  able  to  endure  a  journey 
across  the  open  prairie,  or  the  inhabitant  of  an  open  plain 
may  find  the  passage  through  a  forest  impossible. 

However,  the  great  barrier  is  the  sea,  and  this  has  been 
well  illustrated  in  the  case  of  Bermuda.  Only  certain 
forms  can  make  the  passage  of  this  barrier,  and  therefore 
the  land  fauna  and  flora  of  oceanic  islands  may  differ  from 
those  of  the  nearest  mainland  by  the  absence  of  many 
species,  especially  of  the  higher  animals,  and  oftentimes 
by  the  presence  of  new  forms,  which  have  been  developed 
there,  and  have  not  yet  spread  to  the  mainland.  In  the 
far-away  islands  of  the  mid-ocean  these  peculiarities  are 
very  marked. 


PAET   III.  — THE    OCEAN 

CHAPTER     XII 

GENERAL   DESCRIPTION   OF   THE   OCEAN 

Area  of  the  Ocean.  —  Upon  the  earth  there  are  about 
145,000,000  square  miles  of  ocean  water,  covering  and 
hiding  from  view  about  three-quarters  of  its  surface.  It 
is  not  a  uniform  sheet,  but  is  irregularly  distributed 
in  oceans,  and  between  these  there  are  continents,  while 
above  its  surface  there  rise  numerous  islands.  Its  waters 
bathe  the  shores  of  these  lands,  and  the  contact  between 
land  and  water  is  very  irregular,  especially  in  the  northern 
hemisphere,  where  the  land  is  indented  by  many  bays  and 
harbors,  and  even  by  great  enclosed  seas.  This  line  of 
contact  is  the  scene  of  many  changes  (Chap.  XVIII), 
for  the  waves  of  the  sea  are  incessantly  at  work  in  an 
attack  upon  the  land. 

Importance  of  the  Ocean.  —  The  ocean  surface  is  now 
traversed  by  ships,  carrying  the  products  of  one  zone  to 
the  people  of  another;  but  at  one  time,  before  our  means 
of  navigation  had  reached  such  perfection,  the  sea  surface 
was  a  barrier  to  the  spread  of  man,  almost  as  effectual  as  it 
now  is  to  animals.  Then  distant  lands  were  unknown, 
and  even  short  journeys  by  sea  were  hazardous. 

The  water  of  the  ocean  moderates  the  climate  of  the 
globe,  and  especially  influences  the  land  near  by;  it  fur- 

187 


188  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

nishes  the  air  with  the  vapor  which  falls  as  rain,  and 
therefore  supplies  our  streams  and  lakes  with  water,  which 
after  a  passage  over  the  land,  may  return  to  the  parent 
sea  which  gave  it  birth.  The  rivers  not  only  take  water 
to  the  ocean,  but  also  carry  sediment  from  the  land,  and 
so  the  sea  becomes  the  dumping  ground  for  the  waste  of 
the  land.  This,  added  to  that  which  is  wrested  from  the 
coast  by  the  waves,  is  strewn  over  the  bed  of  the  sea  near 
the  land.  Last,  and  by  no  means  least,  the  ocean  is  the 
home  of  myriads  of  animals,  many  of  which  serve  man 
as  an  important  source  of  food. 

The  Ocean  Water  is  Salt.  —  Unlike  most  of  the  water  of 
the  land,  the  ocean  water  is  distinctly  salt,  and  there  is  a 
great  difference  in  the  percentage  of  this  substance  pres- 
ent in  it.  On  an  average  the  amount  of  salt  in  the  sea 
varies  from  3.3%  to  3.7%  of  the  whole,  so  that  more  than 
96%  of  the  ocean  is  pure  fresh  water.  Yet  so  great  is  the 
bulk  of  sea  water  on  the  earth,  that  the  total  amount  of 
salt  dissolved  in  it,  if  deposited  in  a  layer  over  the  sur- 
face of  the  land,  would  make  a  bed  over  400  feet  thick. 

In  rainy  belts,  such  as  the  doldrums,  the  constant  supply  of  rain 
freshens  the  sea  water;  and  so  also,  the  ocean  is  less  salty  at  the 
mouths  of  large  rivers,  and  near  such  great  glaciers  as  those  of  Green- 
land, which  enter  the  sea  and  furnish  fresh  water  when  the  ice  melts. 
On  the  other  hand,  where  evaporation  is  rapid,  as  in  the  trade-wind 
belt,  the  sea  water  becomes  salter  than  elsewhere;  for  evaporated 
water  is  fresh,  and  when  it  leaves  the  ocean,  the  salt  remains  behind, 
thus  making  the  surface  water  that  remains  still  salter.  Salt  water 
is  heavier  than  fresh,  and  we  say  that  it  is  more  dense.  Calling  the 
density  of  fresh  water  1,  sea  water  has  an  average  density  of  1.02; 
but  as  its  saltness  varies,  the  density  likewise  changes. 

Besides  the  common  salt  which  gives  the  ocean  water 
its  taste,  there  are  minute  percentages  of  other  solids  in 


GENERAL  DESCRIPTION  OF  THE  OCEAN  189 

solution.  The  most  important  of  these  is  carbonate  of 
lime,  the  material  out  of  which  corals  build  their  skeletons, 
and  shellfish  their  shells.  They  take  it  with  their  food 
and  transform  it  to  the  solid  forms  which  we  know  so 
well,  just  as  the  land  animals  take  with  their  food  mate 
rials  which  they  build  into  bone,  teeth,  etc. 

In  addition  to  these  solid  substances,  the  sea  water  car, 
ries  quantities  of  oxygen  ind  other  gases  of  the  atmosphere 
and  the  fishes  take  this  from  the  water  by  means  of  their 
gills,  very  much  as  Ave  take  oxygen  from  the  air  by  mean^ 
of  our  lungs.  Without  it  they  would  die.  Some  of  the 
oxygen  present  in  the  surface  water  is  furnished  by  plants, 
but  much  is  absorbed  from  the  atmosphere,  or  carried  into 
the  sea  by  rain  and  river  water. 

No  one  is  able  to  say  just  what  is  the  source  of  the  salt  in  the  ocean. 
Probably  the  sea  has  always  been  salt,  having  become  so  when  first  the 
waters  gathered  on  the  surface  of  the  globe.  If  the  explanation  of  the 
origin  of  the  earth  which  the  Nebular  Hypothesis  furnishes  is  cor- 
rect, the  sea  in  the  early  history  of  the  globe  must  have  been  impure 
with  many  substances,  for  then  the  earth  was  hot.  As  the  oceans  de- 
scended from  the  atmosphere,  in  the  condition  of  heated  rain,  to  form 
the  great  bodies  of  water  of  the  globe,  they  must  have  contained  salt 
and  other  substances  in  solution.  Upon  this  explanation  much  of  the 
salt  now  contained  in  the  ocean  was  then  furnished  it ;  but  all  rivers 
that  flow  over  the  land  carry  salt,  which  they  have  obtained  from  the 
rocks  and  soils,  and  so  the  sea  is  probably  becoming  Salter  all  the 
time,  just  as  some  lakes  without  outlet  are  even  now  being  trans- 
formed to  salt  seas. 

Temperature  of  the  Ocean  Surface.  —  Naturally  the  tem- 
perature of  the  surface  of  the  ocean  varies  from  place  to 
place,  for  it  is  warmed  by  the  sun  in  a  manner  similar  to 
the  warming  of  the  land.  Near  the  Equator  the  constant 
warmth  produces  warm  ocean  water,  and  in  the  Arctic 


190 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


and  Antarctic  seas,  on  the  other  hand,  the  water  is  cooled 
by  the  constant  coolness  of  the  climate.  However,  the 
temperature  of  the  sea  never  descends  below  the  freezing 
point  of  salt  water,  ^  but  when  this  is  reached  the  water 
congeals  and  sea  ice  is  formed.  Practically  up  to  this  point 
the  water  sinks  as  it  cools,  for  as  in  the  case  of  air,  cooling 
makes  it  more  dense,  and  therefore  causes  it  to  settle. 

The  sea-made  ice  of  the  Arctic,  or  the  Jloe  ice,  as  it  is  called  when 
drifting  about,  forms  over  the  greater  portion  of  the  water  which  sur- 
rounds the  poles.  Far  to  the  north,  the  surface  of  the  ocean  in  winter 
is  transformed  to  solid  ice,  very  much  as  ponds  are  in  the  winter; 


Fk;.  81). 
Arctic  sea  ice,  northern  end  of  Labrador. 

but  the  surface  of  this  is  very  much  rougher,  for  the  winds,  breaking 
th,e  ice,  pile  the  blocks  one  upon  another,  and  the  tides  and  currents 
moving  it  hither  and  thither,  crack  and  break  it  into  blocks,  which 
are  sometimes  flat,  but  commonly  raised  one  upon  another,  so  that 
in  many  places  the  Arctic  ice  is  so  rough  that  travel  over  its  surface 
is  almost  impossible.  The  depth  of  this  sea  ice  is  generally  not  over 
10  or  20  feet,  though  sometimes  greater  than  this. 

Since  the  ocean  waters  are  in  movement  at  the  surface,  this  floe 


1  This  varies  with  the  density  of  the  sea  water,  but  is  generally  between 
28°  and  29°. 


GENERAL  DESCRIPTION  OF  THE  OCEAN  191 

ice  is  carried  about;  and  as  the  movement  of  the  water  is  mainly 
toward  the  south,  the  accumulation  of  the  winter  is  in  part  removed 
to  warmer  latitudes  during  the  summer.  Both  during  winter  and 
summer,  there  is  a  movement  of  the  sea  ice  in  this  direction.  Hence 
it  does  not  increase  in  thickness  as  winter  succeeds  winter,  but  some 
goes  off  to  other  regions,  where,  owing  to  the  warmer  climate,  it  melts 
and  disappears.  There  is  a  constant  procession  of  this  ice  past  the 
shores  of  Baffin  Land  (Figs.  72  and  89),  and  in  spring  and  early 
summer  it  extends  along  the  entire  coast  of  Labrador,  and  even  as 
far  as  Newfoundland. 

The  difference  in  the  temperature  of  the  sea  causes 
movement  to  start,  very  much  as  the  air  moves  by  convec- 
tion. The  warmer  water  of  the  tropics  is  made  light 
by  the  higher  temperature,  and  hence  it  floats,  while 
that  of  the  colder  regions,  being  denser,  settles,  and 
warmer  water  takes  its  place.  This  is  one  of  the  rea- 
sons why  the  ocean  waters  are  in  movement  in  the  form 
of  ocean  currents,  and  it  is  one  of  the  reasons  why  the 
surface  water  changes  temperature  so  slightly ;  for  with 
a  difference  in  temperature,  and  hence  in  density,  the 
water  moves  about,  endeavoring  to  equalize  the  differ- 
ences. 

Life  on  the  Bottom  (Chapter  XI). — For  a  long  time 
almost  nothing  was  known  about  the  bed  of  the  sea,  an 
area  equal  to  about  three-fourths  of  the  earth's  surface. 
It  was  supposed  to  be  a  great  barren  zone  uninhabited  by 
life.  People  knew  that  the  pressure  on  the  bottom  must 
be  tremendous,  and  that  probably  no  sunlight  passed 
through  the  great  depth  of  water,  and  these  peculiarities 
were  thought  to  be  sufficient  reason  for  preventing  the 
existence  of  animals  in  the  depths  of  the  sea. 

When  it  was  found  to  be  possible  to  lay  cables  across 
the  ocean,  it  was  discovered  that  animals  did  live  in  this 


192 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


great  zone,  and  an  interest  was  aroused  among  scientific 
men,  as  a  result  of  which  explorations  of  the  sea  bottom 
were  undertaken,  partly  to  study  animal  life,  and  partly 
to  determine  the  outline  of  the  ocean  bottom,  the  latter 
point  being  necessary  in  some  cases  in  order  to  find  if  it 
would  be  possible  to  lay  cables  upon  it,  and  in  order  to 

select  the  best  lines.  It  is 
now  known  that  animals 
live  there  notwithstanding 
the  coldness  of  water,  the 
darkness,  and  the  great  pres- 
sure; and  it  is  also  known 
that  they  obtain  their  food 
mainly  from  the  death  of 
the  creatures  with  which  the 
surface  waters  teem. 

That  there  is  great  pressure  on 
the  bottom  of  the  ocean  is  proved 
by  the  fact  that  fishes  brought  to 
the  surface  often  have  their  skin 
cracked,  their  eyes  protruding 
from  their  heads,  and  their  air- 
bladders  from  their  mouths. 
While  they  are  on  the  bottom 
there  is  a  tremendous  pressure ; 
but  it  is  exerted  on  every  part  of 
their  body,  j  ust  as  an  air  pressure 
of  about  15  pounds  to  the  square 
inch  is  exerted  on  every  part  of 
our  own  bodies.  But  when  these  creatures  are  raised  to  the  surface, 
the  pressure  is  removed  from  the  outside,  and  that  from  within, 
pressing  outward,  gives  the  results  mentioned.  No  doubt  the  animals 
move  about  in  the  waters  of  the  deep  sea  with  the  same  ease  that 
their  fellow-creatures  do  in  the  waters  at  the  surface. 


FiG.  90. 

Deep-sea  sounding  machine,  with  and 
without  the  sinker 


GENERAL  DESCRIPTION  OF  THE  OCEAN  193 

Methods  used  in  Studying  the  Ocean  Bed.  —  In  the  study  of  the  ocean 
bottom  there  are  several  sets  of  facts  which  are  chiefly  sought.  It  is 
desired  to  know  what  animals  inhabit  these  deep  recesses,  what  the 
temperature  is,  how  deep  the  water  is,  and  hence  something  about 
the  outline  of  the  ocean  bed.  In  carrying  on  the  study,  the  ffrst  thing 
to  be  learned  is  the  depth. 

The  sounding  is  made  by  means  of  a  small  steel  wire  which  is 
reeled  off  from  a  sounding  machine  (Fig.  90).  It  is  necessary  that 
this  shall  be  small,  for  the  weight  of  a  coil  of  heavy  wire,  and  its 
friction  in  the  water,  would  make  it  difficult  to  draw  back  to  the 
surface  what  had  been  lowered.  On  the  end  of  the  sounding  wire 
there  is  a  heavy  iron  ball  which  sinks  to  the  bottom,  and  when  it 
strikes  sends  a  shock  through  the  wire,  which  causes  a  spring  attached 
to  the  machine  to  jump,  and  then  the  reeling  of  the  wire  from  the 
wheel  is  stopped.  So  delicately  made  is  this  machine,  that  the  depth 
is  measured  with  an  error  of  only  a  few  feet,  and  we  now  have  many 
thousand  such  soundings  in  different  parts  of  the  ocean. 

The  sounding  wire  is  so  frail  that  it  would  be  impossible  to  draw 
the  weight  back  to  the  surface  from  the  great  depths,  and  hence  it  is 
left  on  the  bottom.  By  doing  this  it  is  made  certain  that  the  ocean 
floor  has  been  reached.  The  iron  weight,  which  is  a  cannon  ball 
pierced  by  a  hole,  surrounds  a  cylinder,^  at  the  top  of  which  is  a  joint, 
which  when  the  ball  touches  bottom,  bends  and  releases  a  small  hook 
upon  which  the  cannon  ball  has  been  suspended  by  a  wire.  When 
this  hook  drops  down,  —  and  it  cannot  do  so  until  the  bottom  is 
reached,  —  the  ball  is  released,  and  hence  remains  on  the  sea  bed, 
while  the  wire  and  water  bottle  are  drawn  to  the  surface. 

At  the  same  time  the  temperature  of  the  water  is  obtained.  A 
thermometer  is  attached  to  the  sounding  wire  near  the  M'ater  bottle, 
and  others  at  different  points  between  the  surface  and  the  bottom. 
These  are  so  constructed  that  when  the  sounding  wire  is  drawn  in, 

1  This  is  the  loater  bottle,  which  by  automatic  contrivances  remains 
open  on  the  way  down,  and  becomes  hermetically  sealed  by  means  of  a 
tiny  screw  which  revolves  when  the  wire  is  being  drawn  up.  Therefore 
the  water  bottle  brings  to  the  surface  a  sample  of  the  water  from  the 
ocean  bottom.  It  is  worthy  of  mention  that  this  water  is  charged  with 
gases  under  great  pressure,  so  that  when  the  bottle  is  opened  at  the  sur- 
face, the  water  escapes  as  soda  water  does  from  a  fountain. 
o 


194 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


they  turn  upside  down,  being  allowed  to  do  so  by  a  little  screw  wheel 
which  is  unscrewed  by  the  upward  movement  through  the  water. 
When  the  thermometer  overturns,  the  temperature  at  that  particular 
time  is  recorded  by  the  height  of  the  mercury  column,  and  this  is  not 
afterwards  disturbed  unless  the  instrument  is  turned  right  side  up 
again.  Therefore  the  temperature  for  any  depth  may  be  obtained. ^ 
After  a  sounding  has  been  made,  a  dredging  is  often  undertaken 
for  the  purpose  of  obtaining  some  of  the  deep-sea  animals.      The 


Fig.  91. 
Lowering  a  deep-sea  dredge  to  the  ocean  bottom. 

dredge  or  deep-sea  trawl  is  an  iron  frame  with  a  long  bag  net  attached. 
This  is  lowered  by  means  of  a  strong  wire  rope,  and  then  dragged 
over  the  ocean  bottom,  taking  whatever  chances  to  come  in  its  way, 
and  a  dredge  rarely  comes  from  the  bottom  without  containing  some 
of  the  deep-sea  creatures.  It  is  necessary  that  the  frame  shall  be 
dragged  over  the  bottom,  and  to  do  this,  much  more  rope  is  needed 


1  Other  facts  are  also  obtained,  one  of  importance  being  the  determina- 
tion of  the  nature  of  the  materials  covering  the  bottom.  This  is  done  by 
placing  some  soft  substance,  like  soap,  on  the  bottom  of  the  water  bottle, 
and  to  this  the  mud  or  sand  of  the  sea  floor  will  cling  and  be  brought  up 
to  the  surface. 


GENERAL  DESCRIPTION  OF  THE  OCEAN 


195 


than  in  sounding,  for  if  an  extra  amount  were  not  allowed,  when  the 
steamer  began  to  drag  the  dredge  it  would  simply  be  towed  through 
the  water.  Sometimes  a  weight  is  attached  to  the  rope  in  order  to 
cause  it  to  sag  (Fig.  91)  ;  but  in  real  deep  water  the  great  amount  of 
heavy  wire  rope  used  is  sufficient  for  this  purpose. 


NORTH 


-35 


85-36 


Explanation.  ,„™,™, 

36-37^        37-38  m     3S-39vJ 


=n 


Fig.  92. 
Map  showing  temperature  of  the  bottom  of  the  north  Atlantic. 

Ocean  Bottom  Temperatures.  —  As  might  be  expected, 
the  water  of  the  bottom  of  the  sea  is  cold.  Excepting  pos- 
sibly in  the  Arctic  seas,  the  temperature  of  the  water  de- 
creases as  the  depth  becomes  greater.     This  is  because  the 


196' 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


sun  warms  only  the  surface  layers ;  and  whenever,  either 
during  the  winter  or  the  night,  the  water  at  the  surface  is 
cooled,  it  sinks,  because  it  then  becomes  heavier.  So  there 
is  a  settling  of  the  cold  water,  and  therefore  the  tempera- 
ture at  the  bottom  of  the  frigid  seas  is  about  at  the  freezing 
point  of   salt  water.     Between   Iceland  and  Norway  the 

ocean  bottom 
temperature  is 
below  30°;  but 
similar  cold  wa- 
ter is  also  found 
at  the  bottom 
near  the  Equa- 
tor; and  over  a 
great  part  of  the 
sea  floor,  in  its 
deepest  portions, 
the  temperature 
is  between  32° 
and  35°,  even 
within  the  trop- 
ics. Manifestly 
this  cannot  be 
due  to  the  sink- 
ing of  water  in  the  warmer  zones,  for  the  temperature 
of  the  sea  surface  within  the  tropics  is  rarely  below  70° 
(Plate  14).  There  are  various  reasons  for  believing  that 
these  cold  waters  have  come  to  their  place  as  a  result  of  the 
slow  movements  of  ocean  currents  along  the  ocean  bottom, 
from  the  frigid  zones  toward  the  Equator  (Chap.  XIII). 

Generally  the  temperature  of  the  ocean  descends  rather 
rapidly  just  below  the  surface  (Fig.  93),  particularly  in  the 


Fig.  93. 

A  section  of  the  ocean  from  New  York  to  Ber- 
muda, showing  the  temperature  at  various  depths 
(fathom  =  G  feet). 


GENERAL  DESCRIPTION  OF  THE  OCEAN 


197 


tropical  zone,  where  the  upper  layers  of  water  are  warm ; 
but  after  passing  from  this  upper  zone,  the  temperature 
descends  more  slowly.  At  first,  a  depth  of  a  few  hundred 
feet  makes  a  difference  of  several  degrees,  and  then  this 
rate  becomes  less,  until  near  the  bottom  there  may  be  a 
change  of  not  more  than  1°  in  a  thousand  feet.  In  other 
words,  the  ocean  water  is  stratified  into  layers  having 
different  temperatures,  the  highest  at  the  top  and  the 
lowest  at  the  bottom.  Therefore,  as  a  general  rule,  the 
greater  the  depth,  the  lower  the  temperature. 


OCEAN  SURFACE 

^ 

1000  FATHOMS 

GULF  OF  MEXICO 
30.5°        ^--^ 

Z'i.h^,^^^^^ 

^^ 

,^° 

ATLANTIC  OCEAN 
^S^          2000  FATHOMS 

^- -^                                                                                                                                  \.5°                        1 

Fig.  94. 

Section  of  part  of  Gulf  of  Mexico  and  Atlantic  Ocean,  showing  depth  and 

temperature. 

There  are  several  exceptions  to  this,  most  of  which  are 
found  in  such  partly  enclosed  seas  as  the  Mediterranean 
and  Gulf  of  Mexico  (Fig.  94).  In  the  latter,  for  instance, 
the  temperature  decreases  normally  until  it  reaches  about 
39.5°  ;  and  then  there  is  no  further  decrease,  while  outside, 
in  the  open  Atlantic,  where  the  ocean  depth  is  no  greater, 
the  temperature  decreases  to  35°.  Such  a  difference  must 
have  a  special  explanation ;  for  if  there  were  chances  for 
free  circulation,  the  cold  water  of  the  deeper  Atlantic 
would  flow  in  and  displace  the  warmer  water  that  over- 
spreads the  deeper  parts  of  the  Gulf  of  Mexico.     The  ex- 


198  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

planation  for  this  peculiarity  is  found  in  the  fact  that  there 
is  a  rise  in  the  sea  bottom  which  makes  the  bed  of  the  Gulf 
a  basin  with  the  rim  higher  than  the  centre.  The  coldest 
water  that  can  enter  is  that  at  the  level  of  the  top  of  the 
rim,  which  in  the  open  Atlantic  is  about  39.5°. 

The  same  condition  exists  in  the  Mediterranean,  where 
the  temperature  of  the  bottom  at  a  depth  of  12,000  feet  is 
only  5b°^  while  in  the  open  Atlantic  at  the  same  depth 
it  is  20°  lower.  The  temperature  at  the  bottom  of  the 
Mediterranean  is  the  same  as  that  in  the  Atlantic  at  the 
same  level  as  the  bed  of  the  Strait  of  Gibraltar. 

The  Depth  of  the  Sea.  —  The  deepest  part  of  the  sea  so 
far  discovered  lies  to  the  south  of  the  Friendly  Islands, 
which  are  in  the  south  Pacific,  east  of  Australia.  There 
the  depth  is  over  5000  fathoms,  or  30,000  feet,  more  than 
5J  miles,  being  greater  than  the  elevation  of  the  highest 
land  above  the  sea  level,  which  is  about  29,000  feet.  The 
deepest  known  point  of  the  Atlantic  is  4561  fathoms,  within 
70  miles  of  the  island  of  Porto  Rico.  Not  only  are  parts  of 
the  ocean  deeper  below  the  sea  level  than  the  highest  land 
rises  above  it,  but  its  average  depth  is  very  much  greater. 

Near  the  continents,  and  in  some  of  the  partly  enclosed 
seas,  the  ocean  is  not  very  deep;  but  over  nearly  its  entire 
area,  beyond  a  score  or  two  of  miles  from  the  land,  and 
frequently  much  nearer,  the  depth  is  a  mile  or  two,  and 
very  often  more.  Surrounding  most  of  the  continents 
there  is  a  shelf  of  varying  width,  from  a  few  miles  to  over 
100  miles,  over  which  the  sea  is  shallow ;  but  beyond  this 
the  depth  rapidly  inci'eases,  until  the  great  ocean  abysses 
are  reached.  The  best  way  to  gain  an  idea  of  this  differ- 
ence is  to  make  two  sections,  one  from  New  York  to 
Bermuda,  the  other  from  New  York  to  Great  Britain. 


100-500 
500-1000, 


Facing  page  198. 


Plate  15. 
Map  showing  depth  of  the  sea  in 


^.E  OCEAN 


199 


FATHOMS  

1000-2000 I  I 

2000-3000 I  I 

3000-4000 I  I 

Over  4000 |  | 


,tion  of  1000  or  2000 
;onie  to  the  seashore, 
masses  beneath  the  sea 
3  (Fig.  95).  Twenty- 
0  feet,  and  the  water 
distance  of  about  75 
or  600  feet.  Between 
le  ocean-bottom  water 
;  and  cold  in  winter ; 
somewhat  high,  being 
:  Stream. 

continental  shelf;    and 
.dly  increasing,  so  that 


y  depth  and  temperature. 

lie  depth  has  increased 
1  a  mile.  This  region 
\i  at  varying  distances 
d  the  descent  upon  its 
Duntain  slope.  Along 
at  the  depth  of  1000 
ea  becomes  gradually 
thoms  is  reached,  well 
.f  the  sea  is  34°. 
lan  30  miles  away,  the 
in  this  short  distance 
I  stand  on  the  sea  floor 
)fty  conical  mountain, 
jomes  less  on  the  sides 
t  slowly,  then  rapidly, 
)°,  is  reached.     On  the 


PATH 
0-100 


100-500.. 
600-1000.. 


Facing  page 


GENERAL  DESCRIPTION  OF  THE  OCEAN  199 

Starting  from  western  New  York  at  an  elevation  of  1000  or  2000 
feet,  and  passing  over  undulating  ground,  we  come  to  the  seashore, 
where  by  a  further  moderate  descent,  the  land  passes  beneath  the  sea 
and  the  depth  of  the  water  gradually  increases  (Fig.  95).  Twenty- 
five  miles  away  the  depth  is  perhaps  200  or  300  feet,  and  the  water 
continues  to  deepen  very  gradually,  until  at  a  distance  of  about  75 
miles  from  New  York  the  depth  is  100  fathoms,  or  600  feet.  Between 
New  York  and  this  point,  the  temperature  of  the  ocean-bottom  water 
varies  with  the  season,  being  warm  in  summer  and  cold  in  winter ; 
but  at  the  outer  limit  the  temperature  is  always  somewhat  high,  being 
kept  so  by  the  influence  of  the  water  of  the  Gulf  Stream. 

This  zone  of  shallow  water  rests  upon  the  continental  shelf;    and 
going  further  we' find  the  depth  of  the  water  rapidly  increasing,  so  that 


Fig.  95. 

Section  of  ocean  from  New  York  to  Bermuda,  showing  depth  and  temperature. 

within  a  few  miles  from  the  edge  of  the  shelf,  the  depth  has  increased 
from  100  fathoms  to  1000  futlioms,  or  more  than  a  mile.  This  region 
of  steep  slope,  which  borders  the  entire  continent  at  varying  distances 
from  the  coast,  is  called  the  continenfdl  slope,  and  the  descent  upon  its 
face  is  about  as  rapid  as  that  of  a  moderate  mountain  slope.  Along 
this  the  temperature  rapidly  decreases,  until  at  the  depth  of  1000 
fathoms  it  is  38°.  Then  the  depth  of  the  sea  becomes  gradually 
greater,  until  the  deepest  point  of  over  3000  fathoms  is  reached,  well 
toward  the  Bermudas.     Here  the  temperature  of  the  sea  is  34°. 

Within  sight  of  the  Bermudas,  not  more  than  30  miles  away,  the 
bed  of  the  sea  begins  to  rapidly  ascend,  and  in  this  short  distance 
rises  more  than  two  miles,  so  that  if  one  could  stand  on  the  sea  floor 
30  miles  from  the  Bermudas,  he  would  see  a  lofty  conical  mountain, 
quite  like  a  volcano  in  form.  As  the  depth  becomes  less  on  the  sides 
of  this  cone,  the  temperature  increases,  at  first  slowly,  then  rapidly, 
until  the  temperature  of  the  surface,  about  70°,  is  reached.     On  the 


200  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

opposite  side  the  descent  is  similar  to  this,  and  soon  the  cold,  dark 
depths  of  the  sea  are  again  found. 

Passing  from  New  York  toward  England,  the  first  part  of  the 
journey  would  be  similar  to  that  just  described.  The  great  ocean 
depths  that  are  reached  beyond  the  edge  of  the  continental  shelf  are 
nowhere  interrupted  by  shallows,  but  continue  until  near  the  mid- 
Atlantic,  where  the  bottom  rises  in  a  great,  broad  swell,  forming  a 
ridge  which  divides  the  Atlantic  in  a  somewhat  disconnected  way 
from  the  northern  part  to  the  south  Atlantic.  Generally  the  depth 
of  this  mid- Atlantic  ridge  is  1000  or  2l)00  fathoms,  but  the  water  is 
shallower  here  than  on  either  side,  and  upon  its  crest  the  temperature 
is  higher  than  in  the  greater  depths  to  the  east  or  the  west  (Plate  15). 

Beyond  this,  toward  Europe,  the  bottom  again  descends,  reaching 
a  depth  of  nearly  3000  fathoms,  and  then,  at  a  distance  of  50  miles 


OOOVO  m  t^  ^rrt  >P5??! 


C(-,i  crt    rry    m 


CONTINEt^TAL    I    !       ~         n^^l  '     .  1  ......  T>  ',      1 '=9^^''if  "i^**- 


SHELF    ;    ;  ;         <b^     \       sea       ;      level        ,     ^, 


SHELF 


Fig.  9(). 

Section  to  show,  in  diagram,  the  conditions  of  temperature  and  depth  in  the 
Atlantic.    Depth  and  width  of  continental  shelf  greatly  exaggerated. 

from  the  British  coast,  the  continental  slope  rises  steeply,  as  on  the 
American  side ;  and  with  this  rise  the  temperature  increases.  Then, 
from  the  crest  of  this  to  the  shores  of  England,  the  water,  at  first 
about  100  fathoms  deep,  becomes  gradually  shallower ;  and  here,  as 
on  the  American  side,  the  temperature  changes  with  the  seasons. 

Topography  of  the  Ocean  Bottom.  —  These  two  sections 
are  characteristic  of  the  ocean  floor  in  various  parts  of  the 
world.  Bordering  the  continents  there  are  plains  covered 
with  very  little  water,  and  varying  in  width,  terminating 
in  steep  slopes  facing  toward  the  sea,  beyond  which  are 
extensive  plains  covered  by  deep  water  and  extending  over 
nearly  three-fourths  of  the  earth's  surface.  These  plains 
of  the  ocean  bottom  are  the  most  extensive  in  the  world, 


GENEBAL  DESCRIPTION  OF  THE  OCEAN 


201 


forming  great  monotonous  expanses  of  ocean-bottom  clay, 
the  surface  of  which  no  doubt  rises  and  falls  in  gentle 
swells  and  is  here  and  there  relieved  by  single  peaks,  like 
the  Bermudas,   or  groups   of  peaks,    like   the   Hawaiian 


A  part  of  the  Jones  Model  of  the  earth,  showing  a  part  of  the  ocean  bed  and 
the  continents  and  islands.    Copyright,  1894,  by  Thomas  Jones,  Chicago,  111. 

Islands,  some  rising  to  the  surface,  and  some  not  reaching 
so  far.  Here  and  there  also  there  are  mountain  ranges, 
like  the  chains  of  islands  forming  the  East  and  West 
Indies;  and  in  some  parts  of  the  sea,  as  in  the  south 


202 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Pacific,  numbers  of  disconnected  peaks  rise  from  the  great 
ocean-bottom  plain  (Fig.  97). 

There  is  a  distinct  difference  between  the  outline  of  the 


Fig.  98. 
l^orth  and  South  America  in  relief  with  neighboring  ocean  beds— showing 
continent  elevations,  mountain  ranges,  and  ocean  basins.    Copyright,  1894, 
by  Thomas  Jones,  Chicago,  111. 

sea  floor  and  that  of  the  land.  In  both  there  are  volcanoes 
and  mountains,  and  in  both  there  are  plains  and  plateaus; 
but  on  the  land  the  action  of  the  weather  and  the  running 
water  have  gullied  and  carved  the  surface  into  an  exceed- 


GENERAL  DESCRIPTION   OF  THE  OCEAN  203 

ingly  irregular  outline,  not  only  with  great  elevations  and 
depressions,  but  also  with  minute  hills  and  valleys. 

On  the  sea  bottom  however,  these  agents  of  land  ero- 
sion are  absent,  and  the  plains  and  plateaus  are  level, 
while  the  mountains  and  volcanoes  rise  steeply,  with 
smooth,  uncut  sides.  Not  only  are  the  causes  for  the  land 
irregularities  absent,  but  there  is  also  a  constant  rain  of 
sediment,  some  brought  to  the  sea  by  rivers,  some  wrested 
from  the  shore  by  waves,  and  much  formed  by  the  death 
of  animals  which  have  taken  carbonate  of  lime  from  the 
water  and  built  it  into  shells  and  skeletons,  which  when 
they  die  fall  to  the  sea  bottom  and  accumulate.  This 
steady  supply  of  materials,  distributed  over  the  sea  floor, 
tends  to  smooth  out  all  the  smaller  irregularities  which 
naturally  exist.  ,  For  these  reasons  the  main  feature  of 
the  sea  bottom  is  levelness,  broken  here  and  there  by 
steeply  ascending  and  smooth-sided  peaks  and  mountain 
chains  (Plate  15  and  Figs.  97  and  98). 

The  Ocean  Bed.  —  The  land  surface  is  either  bare  rock 
or  soil.  The  sea  bed  is  nearly  everywhere  soft  clay  or 
muddy  ooze.  Near  tlie  land,  gravel  and  sand  are  dis- 
tributed over  the  bottom,  being  furnished  by  rivers  and 
waves ;  but  these  fragments  are  too  heavy  to  be  carried  far 
from  the  coast,  and  they  therefore  settle  near  the  shore, 
so  that  the  further  we  go  from  it,  the  finer  are  the  mate- 
rials covering  the  sea  bed.  Even  100  miles  from  the  coast 
there  are  some  minute  bits  of  clay  floating  in  the  surface 
waters. 

Globigerina  Ooze.  —  But  in  the  open  sea  these  materials 
settle  to  the  bottom  in  very  much  smaller  quantity  than 
the  shells  of  the  minute  and  almost  microscopic  animals 
which  live  in  such  abundance  in  the  surface  waters,  and 


204  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

which  upon  their  death  sink  to  the  bottom.  Therefore 
over  most  of  the  sea  floor  the  bed  is  made  of  an  ooze, 
chiefly  composed  of  remnants  of  these  shells. 

Among  the  shell-bearing  pelagic  animals  some  of  the  most  com- 
mon are  species  belonging  to  the  genus  Globigerina ;  and  hence  over 
great  aieas  this  makes  an  accumulation  of  what  is  called  Globigerina 
ooze,  which  is  somewhat  like  the  chalk  of  England  and  France.  Since 
they  have  been  falling  to  the  sea  bottom  in  the  open  ocean  for  ages 
past,  they  must  have  formed  a  great  thickness  of  this  ooze,  though  to 
make  a  layer  a  foot  in  depth  must  require  many  centuries,  since  each 
grain  represents  the  life  and  death  of  an  animal  whose  size  is  less  than 
that  of  a  pin-head.  In  parts  of  the  ocean  other  minute  species,  such 
as  Diatoms  and  Infusoria,  are  more  common  than  the  Globigerina; 
and  on  the  bottom  of  these  seas  the  ooze  is  called  diatomaceous  or  infu- 
sorial, according  to  which  form  predominates. 

Red  Clay.  —  Covering  even  a  larger  area  than  this  (about  51,000,000 
square  miles),  is  a  still  more  remarkable  deposit  called  red  clay.  This 
is  a  red  mud  occurring  in  the  deeper  parts  of  the  ocean,  below  the 
depth  of  2000  or  2500  fathoms.  In  it  are  found  bits  of  meteoric  iron, 
representing  the  partly  burned-up  meteors,  fragments  of  pumice,  and 
the  more  indestructible  parts  of  sea  animals,  such  as  the  teeth,  which 
are  less  easily  dissolved  than  the  carbonate  of  lime  which  forms  the 
shells.  Because  of  the  presence  of  gases  (especially  carbonic  acid 
gas)  held  under  the  great  pressure  which  exists  there,  the  shells 
have  been  dissolved  in  the  sea  water. 

Every  shell  contains  something  else  than  carbonate  of  lime,  such 
as  minute  quantities  of  iron  and  silica,  which  are  not  so  easily  dis- 
solved as  the  carbonate  of  lime.  Hence,  while  the  latter  is  taken  into 
solution,  these  remain  behind ;  and  it  is  of  this  insoluble  residue  that 
the  red  clay  is  formed,  its  color  being  due  to  the  presence  of  the  iron. 
Therefore,  while  the  Globigerina  ooze  is  very  slowly  formed  by  the 
accumulation  of  many  minute  shells,  the  red  clay  is  gathering  with 
infinitely  greater  slowness  as  a  result  of  the  accumulation  of  minute 
remnants  of  these  tiny  shells. 


CHAPTER   XIII 

THE   MOVEMENTS   OF   THE   OCEAN 

Wind  Waves.  —  A  slight  wind  disturbs  the  surface  of 
the  sea,  causing  ripples  to  start  on  the  water.  Thus  the 
surface  rises  and  falls  as  one  ripple  succeeds  another,  pass- 
ing in  the  direction  toward  which  the  wind  is  blowing; 
but  an  object  floating  in  the  water  does  not  move  along  as 
fast  as  the  tiny  wave  does.  From  this  it  is  seen  that  the 
wave  is  not  a  bodily  forward  movement  of  the  water,  but 
a  disturbance  of  its  surface,  just  as  a  series  of  ring  waves 
move  outward  in  all  directions  from  the  centre,  when  a 
stone  is  thrown  into  the  water.  The  wave  form  consists 
of  two  parts,  an  elevation  and  depression,  the  top  or  ele- 
vated part  being  called  the  event  of  the  wave,  while  the 
depression  between  two  crests  is  called  the  trough. 

The  cause  of  the  wave  is  the  friction  of  the  wind  on  the 
water,  just  as  we  may  raise  a  tiny  wind  wave  in  a  basin 
of  water  by  blowing  over  its  surface.  This  friction  not 
only  causes  the  water  to  rise  and  fall,  but  it  really  does 
drive  a  very  small  amount  forward,  so  that  a  floating  body 
not  merely  rises  and  falls  as  the  wave  passes  under  it,  but 
actually  floats  slowly  forward.  This  surface  movement  of 
the  water  constitutes  a  gentle  current,  a  wind  drifts  at  the 
very  surface.  Therefore  everywhere  that  wind  is  blowing 
over  water,  there  is  a  gentle  current  moving  slowly  along 
before  the  wind. 

205 


206  FIRST  BOOK   OF  PHYSICAL   GEOGRAPHY 

If  the  wind  continues,  and  especially  if  it  freshens,  the 
waves  become  higher,  for  the  cause  is  increased  because 
then  there  is  more  friction. ^  In  addition  to  this,  the 
height  of  the  wave  is  increased  as  one  wave  catches  up 
with  another,  so  that  two  combine  to  form  one  higher  than 
either  of  the  others.  This  increase  may  continue  until 
waves  reach  "mountainous  height,"  which  is  an  exagger- 
ation due  to  the  great  apparent  height  of  the  wave  seen 
from  a  small  ship.^ 

When  a  wind  wave  attains  these  dimensions,  its  effect  is  felt  to  a 
depth  of  200  or  300  feet,  and  it  then  becomes  such  a  powerful  move- 
ment of  tiie  water  that  it  may  last  for  a  long  time  after  its  cause  has 
disappeared.  If  one  throws  a  stone  into  the  water,  the  ripples  which 
it  starts  extend  perhaps  for  scores  of  feet,  and  gradually  die  out;  and 
so  it  is  with  the  great  ocean  waves.  Not  uncommonly,  on  a  perfectly 
calm  day,  when  the  water  surface  is  smooth  and  glassy,  it  heaves  with 
great  swells  or  rollers,  which  have  originated  in  some  distant  part  of 
the  sea,  and  have  passed  far  beyond  the  place  in  which  they  were 
formed. 

In  the  open  ocean  the  waves  are  usually  doing  little 
work  excepting  to  cause  the  surface  to  rise  and  fall. 
Vessels  pass  over  them,  being  lifted  and  lowered  as  the 
waves  pass  by,  but  not  being  injured,  excepting  rarely, 
when  in  violent  gales  the  surface  of  the  sea  is  lashed 
into  a  broken  mass  of  whitened  wave  crests  between  deep, 

1  This  may  be  illustrated  by  blowing  gently  on  the  surface  of  the  water 
in  a  basin,  and  then  blowing  with  much  greater  force. 

2  Large  ocean  waves  rarely  rise  more  than  20  or  .30  feet  from  trough  to 
crest,  but  some  have  been  measured  which  rose  46  feet,  and  it  is  believed 
that  some  reach  the  height  of  60  feet  from  the  lowest  point  of  the  trough 
to  the  highest  part  of  the  crest.  Those  having  a  height  of  40  or  50  feet 
travel  as  much  as  45  miles  per  hour,  and  the  distance  from  the  crest  of 
one  wave  to  that  of  another  may  be  over  700  feet ;  but  one  may  travel  on 
the  sea  for  a  long  distance  without  finding  such  huge  disturbances. 


TBE  MOVEMENTS  OF  THE  OCEAN  207 

narrow  troughs.  Then  small  vessels,  tossed  violently 
about,  are  sometimes  foundered,  and  larger  ones  at  times 
seriously  damaged.  When  they  come  to  the  coast,  the 
waves  change  their  habit,  and  dash  upon  the  exposed 
shores  with  resistless  fury. 

On  the  coast  the  wave  form  is  destroyed,  for  as  the 
water  becomes  shallower,  the  great  waves  have  their  move- 
ment interfered  with,  and  the  motion  of  the  bottom  part 


^A»  DIRECTION  OF  WAVE  MOVEMENT      „    form,. 

'j<       ^•*— "^^  ^^^— ^  ^^--wtfT^L—       ^=t^-^  •••• -  -,. .-  -i  ..J  - -— . 


Fig.  99. 
Diagram  to  show  approach  of  a  wave  upon  a  beach. 

is  partially  checked  as  it  comes  in  contact  with  the  sea 
floor.  The  upper  portion  of  the  wave,  being  less  checked 
in  its  movement,  progresses,  so  that  as  a  result  of  this  the 
crest  gradually  changes,  first  becoming  steeper  on  the  land 
side,  and  then  falling  forward  in  the  form  of  a  breaker^ 
which  rushes  violently  upon  the  coast,  no  longer  as  a  mere 
wave  movement,  but  as  an  onward  flood  of  water,  furi- 
ously hurled  against  the  coast.  Upon  the  beach  the  surf 
rushes  far  above  the  average  water  level ;  and  against  the 
cliffs  the  waves  strike  a  blow  which  causes  the  rocks  to 
tremble,  and  sends  a  roar  through  the  air,  while  the  spray 
[lashes  high  upon  the  coast. 

During  these  times  of  violent  waves  there  is  great  work 
of  destruction  being  done.  The  sand  of  the  beach  is 
washed  backward  and  forward,  or  the  pebbles  are  rolled 
about,  causing  a  deafening  roar  as  they  are  ground  together. 
This  constant  grilldii:ig'  ^ve^^rs  Ih^ui  slowly  away,  rounding 


208  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

them  and  finally  reducing  them  to  bits  of  clay  or  sand> 
The  beaches  are  the  mills  in  which  the  rock  fragments, 
driven  ashore  by  the  waves,  or  wrested  by  them  from  the 
rocky  headlands,  are  ground  to  form  the  sediment  which 
is  being  strewn  over  the  ocean  bed.  The  violent  waves 
not  only  work  here,  but  also  against  the  cliffs  of  hard 
or  soft  rock,  which  they  are  also  wearing  slowly  away. 
Sometimes  the  power  of  the  waves  becomes  so  great,  that 
blocks  tons  in  weight  are  wrested  from  their  beds  and 
moved  along  the  coast. 

But  if  the  waves  merely  destroyed  the  coast,  they  would 
soon  lose  their  power  to  cut  into  the  land,  for  as  the  cliffs 
crumbled,  and  the  debris  accumulated  at  their  base,  they 
would  be  protected  from  further  attack,  and  the  waves 
would  expend  their  energy  on  the  fragments  which  they 
had  wrested  from  the  land.  Some  of  these  must  be 
removed^  so  as  to  leave  the  cliff's  open  to  the  further 
attack  of  the  waves.  As  the  boulders  and  pebbles  are 
ground  to  pieces  by  the  waves,  particles  are  worn  off  which 
are  so  fine  that  they  can  float  away  in  the  moving  water. 
The  tidal  currents  help  in  this  removal,  and  so  also  do 
the  wind-formed  currents  of  surface  water  which  move 
before  the  winds. 

Then  also,  when  the  waves  come  upon  the  coast  diago- 
nally, as  they  often  do,  the  surf,  instead  of  running  di- 
rectly along  the  coast,  passes  not  only  toward  the  land, 
but  for  some  distance  along  its  margin^  so  that  as  wave 
follows  wave,  the  sand  and  pebbles  are  washed  along  the 
shores  in  the  surf.     Fragments  are  thus  carried  over  con- 

1  So  rapid  is  this  work  of  destruction  that  bricks  which  have  been 
washed  ashore  upon  the  beach,  are  ground  to  tiny  pebbles  in  the  course  of 
a  few  years. 


TBt:  MOVEMENTS  OF  THE  OCEAN  209 

siderable  distances  in  the  direction  toward  which  the 
waves  move.  Also  when  the  waves  run  upon  the  beach 
in  the  form  of  surf,  the  water  must  return  to  the  sea. 
This  return  movement,  which  begins  when  the  wave  has 
worn  itself  out  against  the  shore,  and  is  interrupted  when 
the  next  wave  comes,  continues  below  the  immediate  sur- 
face in  the  form  of  a  current  along  the  bottom.  This, 
which  is  known  as  the  undertow^  is  an  outward-moving 
current,  flowing  with  such  force  that  bathers  are  some- 
times caught  in  it  and  drawn  down  and  held  near  the 
bottom  until  life  is  extinct.^ 

By  these  several  means  the  particles  which  the  waves 
take  from  the  coast  are  slowly  ground  up  and  carried 
away,  some  out  to  sea,  where  they  settle  in  the  more  quiet 
water,  and  some  along  shore,  until  an  indentation  is 
reached,  where,  being  driven  into  the  more  quiet  water  of 
the  protected  bay,  it  settles  to  the  bottom.  While  it  is 
necessary  for  much  of  the  material  to  be  carried  away,  in 
order  that  the  waves  may  continue  their  attack  upon  the 
land,  it  is  also  important  that  some  fragments  should  also 
remain  in  their  grasp ;  for  the  work  of  the  waves  is  made 
effective,  not  so  much  by  the  direct  action  of  the  water,  as 
by  the  battering  of  the  pebbles  and  sand  which  they  hurl 
against  the  shore.  These  are  the  tools  with  which  the 
wave  does  its  work. 

This  attack  of  the  waves  produces  different  results 
according  to  their  violence,  the  exposure  of  the  coast,  its 
form,  or  its  rock  structure ;  and  it  is  performing  a  great 
work  of  change,  as  a  result  of  which  the  coasts  of  the 

1  When  swimming  in  uxb  uoco,*:  S"rf  it  is  necessary  to  keep  near  the 
surface,  and  if  the  feet  are  allowed  to  sink  toward  iiJlG  ibc^t'toni  the  entire 
body  may  be  drawn  under. 


210 


FIRST  BOOK  OF  PHYSICAL  GEOGRAFBT 


world  are  not  only  varying  in  outline,  but  are  being  con- 
structed into  certain  definite  forms.  The  study  of  these 
changes  may  be  deferred  until  we  have  gained  a  little 
more  knowledge  of  the  land  (see  Chap.  XVIII). 

The  Tides  :  ^  Nature  of  the  Tides.  —  In  most  parts  of  the 
ocean  the  water  surface  rises  twice  each  day.  The  water 
slowly  advances,  and  a  person  standing  upon  the  coast  is 
driven  from  his  position  as  it  rises.     Then  for  a  little 

more  than  6  hours,  it  retires  as 
slowly  as  it  came,  when  it  again 
begins  to  rise;  and  this  is  re- 
peated again  and  again,  so  that 
every  12  hours  and  25  minutes 
there  is  a  high  tide,  with  low 
tide  between.  The  rising  tide 
is  called  the  flow;  the  falling, 
the  ehh.  If  one  watches  the 
tide  with  care,  he  sees  that  it 
does  not  rise  to  the  same  level 
day  after  day,  but  that  there  is 
considerable  difference  in  its 
height. 

Again,  from  one  place  to  an- 
other there  is  a  variation  in  the 
height  to  which  the  tide  rises. 
At  Key  West,  and  on  oceanic  islands,  the  rise  is  only  2 
or  3  feet ;  in  Hudson  Straits,  north  of  Labrador,  it  reaches 
an  elevation  of  30  feet;  in  parts  of  the  Bay  of  Fundy  the 
high  tide  is  often  30  or  40  feet  above  the  low,  and  in 

1  From  necessity  this  subjert  is  tiCaUju  'urlei^j  liere.  The  teacher  who 
wishes  to  exprtlm  me  subject  will  find  some  material  for  this  in  my  Ele- 
mentary Physical  Geography,  as  well  as  in  other  books. 


FiG.  100. 

Diagram  to  illustrate  the  dis- 
tortion of  the  ocean  by  the 
attraction  of  the  moon,  the 
distortion  being  of  course 
greatly  exaggerated. 


TBE  MOVEMENTS  OF  THE  OCEAN 


211 


some  places  is  said  to  reach  50  or  60  feet.  In  Ungava 
Bay,  in  the  northern  part  of  the  Labrador  peninsula,  the 
tide  rises  about  as  high  as  it  does  in  the  Bay  of  Fundy. 

Causes  of  Tides.  — ■  From  an  examination  of  the  tides 
it  is  evident  that  they  are  waves  of  rising  and  falling 
water,  and  we  know 
that  all  ocean  shores 
are  disturbed  by  them. 
The  tides  reach  greater 
height  in  V-shaped 
bays  than  on  exposed 
oceanic  islands,  and 
this  is  evidently  due 
to  the  influence  of  the 
land.  When  a  wind 
wave  approaches  the 
beach,  it  is  caused  to 
change  its  habit  and 
to  reach  higher  than 
the  natural  level  of  the 
sea.  It  is  piled  up; 
and  so  is  the  tide  wave 
as  it  enters  the  nar- 
rowing bay. 

If  we  observe  the 
variation  in  height  of 
the  tide  at  any  single  place,  we  find  that  its  change  cor- 
responds with  the  variations  in  the  phases  of  the  moon, 
and  this  would  lead  us  to  believe  that  in  some  way  the 
tide  is  caused  by  the  moon. 

The  moon,  the  nearest  of  the  heavenly  bodies,  is  at  all 
times  exerting  a  pull  upon  the  earth,  just  as  this  is  upon 


Fig.  101. 

Diagram  to  show  advance  of  tidal  wave  in 
the  Atlantic.  Figures  represent  hours  of 
the  day ;  heavy  lines,  noon. 


:2l2 


FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 


® 


© 


the  moon.  This  pull  is  the  attraction  of  gravitation,  by 
which  the  bodies  of  the  solar  system  are  bound .  together, 
and  it  is  something  like  that  which  holds  the  air  to  the 
earth  and  causes  a  stone  to  fall  to  the  ground.  If  the 
moon  and  earth  could  cease  revolving,  they  would  come 
together  as  certainly  as  a  stone  will  fall  to  the  earth  if  it 

is    dropped  from 
(T\  ^ —  the  hand.  There- 

fore the  moon  is 
endeavoring  to 
draw  the  earth 
toward  it,  and  to 
a  slight  extent  is 
really  succeeding 
in  drawing  the 
liquid  part  of  the 
earth.  Therefore 
a  wave  is  raised 
in  the  ocean,  and 
as  the  moon  pass- 
es through  the 
heavens,  the  tide 
wave  follows  it. 
By  this  attrac- 
tion one  wave  is 
formed  under  the  moon,  and  one  on  the  opposite  side; 
but  they  do  not  pass  around  the  earth  directly  under  the 
moon,  for  they  lag  behind,  and  hence  follow  it. 

What  the  moon  is  doing  in  this  respect,  the  8un  is  nls:o 
doinsr :  but  this  body,  though  much  larger  than  the  moon, 
is  so  much  further  from  us'  that  its  influence  is  less, 
because  the  attractive  force  varies  not  only  with  the  mass 


(5) 


© 


Fig 


Thres  diagrams  to  show  (A)  the  sun,  earth,  and 
moon  in  line  at  new  moon ;  (B)  the  same  at  full 
moon ;  and  (C)  the  moon  and  sun  in  opposition 
during  the  quarter.  In  the  first  two  cases  the 
waves  of  moon  and  sun  are  formed  in  about  the 
same  place ;  but  in  the  third  they  are  formed  in 
different  places  and  hence  the  tidal  rise  is  less. 


THE  MOVEMENTS   OF  THE  OCEAN  213 

of  the  body,  but  also  with  its  distance.  Therefore  four 
waves  are  formed,  the  two  larger  ones  by  the  moon,  and 
two  much  smaller  waves  by  the  sun.  At  new  and  full 
moon,  the  sun,  earth,  and  moon  are  nearly  in  line,  and 
then  both  the  lunar  and  solar  tides  are  raised  in  about  the 
same  place.  Then  the  two  combine  to  form  a  larger 
wave  than  usual,  giving  the  spring  tide.  When  the  moon 
is  in  the  quarter,  the  sun  and  moon  are  exerting  their 
influence  in  directions  nearly  at  right  angles  to  one  an- 
other, and  the  two  waves  are  therefore  somewhat  in  oppo- 
sition, so  that  a  lower  or  neap  tide  is  caused.  Therefore, 
as  the  phases  of  the  moon  change,  the  height  of  the  tide 
varies. 

There  are  other  causes  for  variations,  one  of  the  most 
important  being  the  difference  in  distance  of  the  moon. 
This  body  travels  around  the  earth  once  in  a  lunar  month, 
not  in  a  circle,  but  along  an  elliptical  path  with  the  earth 
at  one  of  the  foci.  Therefore  at  one  part  of  the  lunar 
month,  the  moon  is  much  nearer  than  at  the  other  times. 
When  nearest  to  us  the  moon  is  said  to  be  in  perigee,  and 
when  furthest  in  apogee.  The  attraction  varies  greatly 
with  the  distance  of  the  body ;  and  hence  when  the  moon 
is  in  perigee,  the  tide  is  made  high,  particularly  if  the 
moon  is  full  or  new.^ 

Effects  of  the  Tides.  —  Sometimes  the  tides,  instead  of  being  merely 

the  rising  and  falling  of  water  in  true  wave  form,  become  real  cur- 

nts.     Then  navigation  is  checked  or  aided,  sand  and  clay  are  drifted 

about,  bars  '^  -    ''  ■■'^  ^^'  the  mouths  of  harbors,  and  the  waves  are 

1  An  especially  vaiuabie  lesson  in  tide  variation  may  o^  ^^ven  by  a 
careful  study  and  plotting  of  the  tide  height  at  various  places,  as  given  hi 
the  Tide  Tables  for  the  Atlantic,  published  by  the  United  .States  Coast 
Survey,  Washington,  D.C. ;  price  25  cents. 


214  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

aided  in  moving  the  sediment  either  along  the  coast  or  else  out  to  sea. 
These  currents  are  caused  by  the  approach  of  the  wave  over  the  shal- 
low bottom,  in  a  manner  similar  to  the  approach  of  the  wind-wave 
surf  on  the  beach.  As  the  tide  rises,  the  current  moves  one  way,  and 
the  outgoing  tide  in  the  opposite  direction.  Sometimes  these  tidal 
currents  are  so  strong  that  navigation  is  impeded,  and  oftentimes  it 
is  impossible  to  row  a  boat  against  the  current.  In  parts  of  the  Bay 
of  Fundy,  where  the  tide  rises  to  such  great  height,  the  currents  or 
races  become  as  violent  as  a  rapid  river  current,  and  it  is  unsafe  to 
attempt  to  navigate  these  waters  unless  one  is  perfectly  familiar  witli 
all  the  peculiarities  of  movement.  There  the  tide,  advancing  over 
the  low  mud  flats,  moves  as  rapidly  as  a  man  can  run. 

There  are  many  special  causes  for  these  currents.  Sometimes  the 
tide  rises  higher  on  one  side  of  a  peninsula  than  on  the  other,  and 
then,  if  there  is  a  strait  between  the  two  bays,  with  the  rising  and 
falling  tide  the  water  moves  backward  and  forward  through  it. 
Again,  after  passing  through  a  nan-ow  arm  of  the  sea,  the  water  may 
enter  a  broad  bay;  and  then,  when  the  tide  falls,  this  great  mass  of 
water  rushes  out  through  the  narrow  channel  with  the  velocity  of  a 
river.  On  the  other  hand,  the  tide  may  sometimes  lose  its  height  and 
velocity,  when  after  passing  through  a  narrow  entrance  it  reaches 
a  broad  sea,  as  the  tide  wave  does  after  passing  through  the  Strait 
of  Gibraltar  into  the  Mediterranean,  where  there  is  almost  no  rise 
and  fall  of  the  tide.^ 

This  silent  and  regular  rising  and  falling  of  the  ocean 
surface  is  one  of  the  most  interesting  features  connected 
with  this  great  expanse  of  water.  On  the  open  sea  one 
might  travel  for  thousands  of  miles,  never  knowing  of 
its  existence ;  but  along  the  land  margin  it  becomes  very- 
apparent  and  important. 

1  Here,  howevpr^  ^  separate  tide  of  small  size  is  generated  by  the  same 
^S^iiSS-^vclaich  makes  the  great  ocean  tidal  waves.     In  fact,  even  in  large 
lakes  there  are  slight  tides  of  this  origin,  besides  the  more  irregular  fluct- 
uations of  the  surface  due  to  winds  and  changes  in  the  air  pressure,  and 
known  under  the  name  of  seiches. 


±\icing  page  21k. 


Fla 
Diagrammatic  cha 


Ban  currents. 


THE  MOVEMENTS   OF  THE   OCEAN  215 

Ocean  Currents  :  Differences  in  Temperature  —  When 
speaking  of  the  temperature  of  the  sea  (Chap.  XII),  it  was 
stated  that  there  are  reasons  for  believing  that  there  is  a 
circulation  in  the  ocean,  consisting  of  sinking  water  in 
the  colder  latitudes,  and  rising  in  warmer  belts,  with  bot- 
tom currents  moving  toward  the  Equator,  and  surface 
movements  away  from  it.  This  conclusion  seems  neces- 
sary as  a  result  of  the  fact  of  greater  warmth  in  the  one 
place  than  in  the  other,  and  there  are  other  facts  pointing 
to  the  same  conclusion.  The  temperature  of  the  deep  sea 
can  be  accounted  for  only  on  this  explanation.  Moreover, 
if  there  were  no  such  circulation,  how  could  the  deep-sea 
animals  obtain  the  oxygen  which  they  need?  If  the  waters 
were  quiet,  these  creatures  would  have  no  source  for  this 
necessary  element;  but  a  slow  circulation  along  the  bot- 
tom, supplied  from  the  surface,  would  furnish  it. 

On  this  theory  the  ocean  is  somewhat  like  the  air,  and 
the  great  movements  of  ocean  currents  are  similar  to  the 
circulation  of  the  atmosphere;  but  there  is  one  very 
important  difference :  the  air  is  warmed  from  below,  and 
being  made  lighter  near  the  ground,  rises  in  the  warmer 
belts,  while  it  cools  and  settles  in  the  colder  regions.  But 
the  sun's  heat  does  not  reach  to  the  bottom  of  the  sea,  and 
the  warming  of  this  is  therefore  confined  to  the  surface 
layers ;  hence  the  comparison  with  the  air  is  not  strictly 
correct.  There  are  actual  movements  of  the  ocean  similar 
to  those  which  this  theory  demands,  but  they  seem  to  be 
more  powerful  than  this  cause  alone  could  produce ;  and 
although  nearly  every  one  believes  that  differences  in  tem- 
perature cause  a  slow  circulation  of  cold  water  along  the 
ocean  bottom,  and  aid  in  the  production  of  some  of  the  cur- 
rents at  the  surface,  another  explanation  seems  to  be  neces- 


216  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

sary  for  the  more  powerful  of  the  surface  currents.  Before 
stating  this  theory  let  us  look  briefly  at  the  actual  condi- 
tions of  oceanic  circulation. 

Atlantic  Currents.  — For  illustration  of  these  ocean  move- 
ments we  may  select  the  north  Atlantic,  in  which  the 
ocean  currents  are  as  well  developed  as  anywhere,  and 
much  more  carefully  studied.^  There  is  a  slow  circula- 
tion of  the  sea,  both  north  and  south  of  the  Equator,  mov- 
ing toward  the  belt  of  calms  in  the  direction  followed  by 
the  trades,  —  that  is,  southwest  on  the  northern  side  of  the 
Equator,  and  northwest  to  the  south  of  it.  In  the  dol- 
drums, between  the  trade-wind  belts,  this  drift  of  water 
moves  westward  until  the  coast  of  South  America  is 
reached,  where  it  divides  into  two  unequal  parts,  the  larger 
journeying  northward  along  the  northern  coast  of  South 
America,  the  smaller  southwards.  The  northern  portion 
passes  as  a  slow  drift  of  water,  partly  into  the  Caribbean, 
partly  between  the  West  Indies,  and  partly  outside  of 
these  islands  in  the  open  sea.  The  latter,  turning  to  the 
right  under  the  influence  of  the  earth's  rotation,  circles 
eastward,  then  southeastward,  and  finally,  passing  along 
the  coast  of  Europe  and  northern  Africa,  again  comes  within 
the  zone  of  the  trade  winds.  Hence  it  circles  around, 
forming  a  great  eddy  of  slowly  moving  surface  water, 
and  this  is  found  in  all  the  oceans  that  are  crossed  by  the 
Equatorial  belt,  turning  to  the  left  in  the  southern  seas 
and  to  the  right  in  the  northern. 

That  part  of  the  Equatorial  drift  which  passes  into  the 
Caribbean  Sea,  circles  through  it,  becoming  warmer,  enters 

1  Although  some  of  the  conditions  on  the  ocean  bottom  have  been 
investigated,  it  has  not  been  found  possible  to  determine  the  rate  of  move- 
ment of  the  cold  bottom  v^ater. 


THE  MOVEMENTS   OF  THE  OCEAN 


217 


the  Gulf  of  Mexico,  and  passes  out  of  it  between  the  end 
of  Florida  and  Cuba,  where  it  emerges  as  the  Gulf  Stream 
(Fig.  103).  This  current,  flowing  rapidly  at  first,  as  a 
narrow  stream,  loses  velocity  as  it  passes  along  and  becomes 
broader.  Off  the  Florida  coast  it  is  a  distinct  stream  in 
the  sea,  flowing  at  the  rate  of  four  or  five  miles  an  hour ; 
but  by  the  time  the 
latitude  of  Cape 
Cod  has  been 
reached,  its  veloc- 
ity is  reduced  to 
less  than  two  miles 
an  hour.  For  a 
while  it  flows  near 
the  American 
coast,  then,  about 
in  the  latitude  of 
Cape  Hatteras,  it 
slowly  turns  to  the 
right,  leaving  the 
American  shores 
and  crossing  the 
Atlantic  to  Europe, 
being  deflected  in 
this  direction  by 
the  earth's  rota- 
tion. Against  the 
European  coast  it  divides,  some  turning  southwards  toward 
the  Equator,  and  some  going  northward  past  Scandinavia 
into  the  A  re  tic. ^ 

1  Nansen  has  proved  that  there  is  a  current  in  the  Arctic  from  the 
northern  shores  of  Asia  to  the  region  between  Greenland  and  Spitzbergen. 


Fig.  103. 

Diagram  to  show  the  currents  of  the  eastern 
north  Atlantic.  Figures  tell  rate  of  movement 
in  miles  per  hour. 


218  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHT     . 

In  the  north  Atlantic  there  is  a  cold  current  called  the 
Labrador  current,  flowing  from  the  Arctic.  This  comes 
down  from  the  north,  between  Greenland  and  Baffin  Land, 
past  Labrador,  and  as  far  as  New  England,  where  it  dis- 
appears in  the  Gulf  of  Maine.^  It  hugs  the  American 
coast  closely,  being  turned  to  the  right  by  the  deflective 
eff,ect  of  the  earth's  rotation. 

Aside  from  similar  currents  in  the  several  oceans,  there 
is  a  drift  of  water  in  the  southern  hemisphere,  south  of 
Africa,  South  America,  and  Australia,  where  the  water 
moves  in  the  direction  followed  by  the  prevailing  wester- 
lies. These  movements  constitute  the  great  ocean  cur- 
rents of  the  globe,  and  as  a  result  of  them  the  waters  of 
the  sea  are  almost  constantly  in  motion  in  certain  definite 
directions. 

The  Explanation.  —  It  will  be  noticed  that  where  they 
start,  the  currents  have  the  same  directions  as  the  pre- 
vailing winds,^  and  this  is  believed  by  many  to  be  the 
most  prominent  cause  for  the  surface  ocean  currents. 
Water  is  always  drifted  before  the  wind,  and  small  cur- 
rents of  this  origin  may  be  seen  along  the  coast,  not  only 
in  the  ocean  but  in  many  lakes.  Drifted  along,  turned  by 
the  land,  and  by  the  effect  of  rotation,  the  water  circles 
through  the  seas,  giving  the  great  oceanic  circulation. 

No  doubt  there  is  also  a  great  but  slow  movement 
caused  by  differences  in  temperature;  but  this  seems  to  be 
a  less  important  cause  for  surface  currents  than  the  winds. 

1  The  gulf  enclosed  between  Nova  Scotia  and  Cape  Cod. 

2  The  Labrador  current  is  possibly  started  by  the  north  winds,  but 
probably  is  a  partial  return  of  the  water  that  is  sent  into  the  Arctic  by  the 
Gulf  Stream,  and  the  fresh  water  that  enters  the  Arctic  from  the  great 
north-flowing  rivers  of  Asia  and  North  America.  Even  the  Gulf  Stream 
is  aided  in  crossing  the  Atlantic  by  the  prevailing  westerlies. 


TBt!  MOVIJMENTS  OF  THE  OCEAN  219 

There  are  several  reasons  for  this  conclusion,  but  the  most 
prominent  one  is,  that  the  ocean  currents  are  less  pro- 
nounced in  the  southern  than  in  the  northern  oceans ;  yet 
southern  oceans  are  open  to  the  cold  Antarctic,  while 
those  north  of  the  Equator  are  more  nearly  closed  to  the 
cold  Arctic  waters ;  and  particularly  is  this  true  of  the 
north  Pacific.  If  the  oceanic  circulation  is  chiefly  due  to 
differences  in  temperature,  it  should  be  most  pronounced 
in  the  southern  oceans,  where  there  is  a  greater  chance  for 
this  difference  to  express  itself ;  but  the  reverse  is  true. 

Effects,  —  Ocean  currents  are  chiefly  important  in  modi- 
fying the  climate.  By  them  the  ocean  itself,  and  the 
neighboring  lands,  are  made  either  Avarmer  or  cooler.  The 
currents  do  not  cut  the  shores  as  do  the  waves,  nor  are 
they  moving  rapidly  enough  to  carry  much  sediment,  as 
are  some  of  the  tidal  currents ;  but  they  are  performing 
a  great  woi-k  in  nourishing  large  numbers  of  marine  ani- 
mals, which  float  in  these  waters,  and  which  furnish  food  to 
the  larger  animals.  The  warm  ocean  currents  are  food 
bringers  for  the  colonies  of  corals  which  exist  in  the 
warmer  portions  of  the  ocean,  and  they  therefore  aid  in 
the  building  of  many  lands  in  the  sea.  The  Bahamas, 
the  Bermudas,  the  southern  end  of  Florida,  and  the  multi- 
tudes of  islands  in  the  south  Pacific,  are  coral  islands 
made  from  the  skeletons  of  creatures  nourished  in  the 
warm  water  of  the  ocean  currents. 


PAET   IV.  — THE    LAND 

CHAPTER   XIV 

THE   EARTH'S   CRUST 

Condition  of  the  Crust.  ^  —  Nearly  everywhere  at  the 
very  surface  of  the  land  there  is  a  soil  covering,  beneath 
which,  at  depths  usually  not  more  than  a  few  feet,  though 


Fig.  104. 
A  rock  made  of  horizontal  layers  of  different  kinds. 

sometimes  200,  300,  or  even  400  feet,  there  is  solid  rock, 
and  this  continues  to  as  great  a  depth  as  man  has  pene- 

1  At  this  point  I  would  suggest  a  review  of  a  part  of  Chapter  I,  particu- 
larly pages  7-13. 

220 


THE   EABTH'S  CRTTST  221 

trated  into  the  crust.  These  rocks  are  of  many  different 
kinds,  and  when  seen  in  a  river  gorge,  or  any  other  cut- 
ting, they  are  usually  found  to  be  in  layers,  and  in  many 
cases  these  are  arranged  one  on  another,  generally  in  hori- 
zontal beds  (Figs.  104,  136,  and  139),  but  sometimes  in 
layers  tilted  at  various  angles  (Chapter  XIX),  perhaps 
even  vertically.  Some  of  these  are  made  up  of  fragments, 
such  as  grains  of  clay  or  sand,  some  of  shells  of  animals, 
forming  limestones,  and  some  are  made  of  distinct  min- 
erals, and  these  are  called  crystalline  rocks. 

Minerals  of  the  Crust :  ^  Elements.  —  When  subjected  to 
chemical  analysis,  it  is  found  that  in  any  rock  there  are 
certain  elements. ^  Chemists  have  so  far  discovered  about 
70  elements,  but  only  a  very  few  are  really  common 
in  the  earth,  the  three  most  abundant  being  oxygen,  sili- 
con, and  aluminum;  and  the  others,  that  are  less  abundant, 
though  still  important  in  the  crust,  are  iron,  calcium, 
magnesium,  potassium,  sodium,  carbon,  and  hydrogen. 
Oxygen  forms  47%  of  the  earth's  crust,  silicon  27%,  and 
aluminum  8%,  so  that  these  three  constitute  about  82%  of 
the  known  rocks  of  the  earth's  crust.     In  many  of  the 

1  No  attempt  can  be  made  here  to  treat  the  subject  of  mineralogy. 
A  very  good  elementary  book  is  Dana's  Minerals  and  How  to  Study 
Them  ;  and  in  his  larger  works  will  be  found  a  more  complete  treatment. 
The  only  way  for  students  to  really  understand  minerals  is  to  examine 
them  and  study  their  characteristics  from  actual  specimens.  I  would 
suggest  the  introduction  of  some  simple  laboratory  work  in  the  study  of 
minerals,  preferably  taking  up  only  a  dozen  or  twenty  of  the  most  com- 
mon, which  are  selected  not  so  much  for  crystal  perfection  as  to  show  the 
other  characteristics. 

2  An  element  is  the  simplest  form  to  which  man  has  been  able  to  reduce 
matter.  Gold,  for  instance,  cannot  be  made  simpler,  nor  can  mercury, 
nor  the  oxygen  of  the  air ;  but  each  of  these  may  be  made  to  combine 
with  other  elements  to  form  some  chemical  combination. 


222 


FIRST  BOOK  OF  PHYSICAL  GEOGRAPBT 


rocks,  such  as  those  made  of  clay,  the  elements  are  often 
in  complex  and  indefinite  combination ;  but  frequently  they 
are  present  in  distinct  combinations,  known  as  minerals. 

Definition.  —  A  mineral  may  be  defined  as  a  homogeneous 
solid  of  definite  chemical  composition^  occurring  in  nature^  but 
not  of  apparent  organic  origin.  There  are  perhaps  2000 
minerals  known,  but  most  of  them  are  so  rare  that  they 
are  found  only  in  the  larger  mineral  collections.  One 
or  two  hundred  may  be  considered  fairly  common,  but 
less  than  a  dozen  are  really  important  in  the  rocks. 

Quartz.  — •  By  far  the  most  abundant  of  these  is  quartz^ 
a  mineral  composed  of  the  two  elements  silicon  and  oxygen 

(SiOg),  and  found  in  all 
granites  and  sandstones, 
as  well  as  in  many  other 
rocks  and  most  soils.  It 
is  so  hard  that  it  cannot 
be  scratched  with  a  knife, 
and  it  will  cut  glass. 
Very  often  it  is  found  in 
perfect  crystalline  form, 
being  bounded  by  smooth 
glassy  planes,  which  form 
a  six-sided  prism,  while  on  one  end,  or  perhaps  on  both^ 
there  are  hexagonal  pyramids.  More  commonly  the  quartz 
crystal  is  less  perfect,  and  in  fact  most  of  the  quartz  of 
the  world  occurs  in  grains,  generally  of  small  size. 

Quartz  may  be  told  from  the  single  grains  of  other  com- 
mon minerals  by  its  hardness,  and  by  its  glassy  surface, 
which  reflects  light  as  glass  does;  and  for  this  reason  it  is 
said  to  have  a  glassy  lustre.  It  resembles  glass  also  in  its 
fracture,  for  when  it  breaks  it  has  a  shelly,  rounded  sur- 


Quartz  crystal,  showing  three  of  the  prism 
sides  and  three  of  the  pyramid  faces. 


THE  EARTH'S   CRUST  223 

face  quite  like  broken  glass.  This  is  called  the  conchoidal 
or  shelli/  fracture.  Being  hard,  quartz  tends  to  make 
rocks  durable,  for  it  is  not  easily  -ground  down  by  the 
agents  which  are  cutting  rocks  (Chapter  XV).  It  is 
more  durable  than  other  minerals  for  another  reason, — 
when  exposed  to  the  air  it  does  not  change  and  crumble, 
as  some  minerals  do,  but  throughout  all  the  attacks  of  the 
weather  remains  pure,  clear  quartz.  Quartz,  when  com- 
pared with  the  mineral  next  described,  is  about  as  gold  is 
to  iron.  In  the  air  gold  remains  fresh,  but  iron  soon  rusts 
and  becomes  soft.  Quartz  is  also  slightly  soluble,  and  the 
water  flowing  over  the  land  always  carries  small  quanti- 
ties of  it;  but  in  this  respect  it  is  much  less  soluble  than 
calcite,  and  more  so  than  feldspar.  Therefore  in  most 
respects  this  is  a  very  durable  mineral. 

Feldspar.  —  This  mineral  is  another  exceedingly  impor- 
tant component  of  the  crust.  With  quartz  it  occurs  in 
granite,  and  this  rock  is  chiefly  made  of  these  two  miner- 
als. Feldspar  sometimes  occurs  in  crystals,  though  much 
more  rarely  than  quartz.  When  it  is  found  in  gi-ains 
together  with  quartz,  the  two  can  generally  be  told  apart 
by  the  fact  that  feldspar  is  duller  and  whitish,  and  has  a 
much  less  glassy  lustre.  Besides  this,  a  fractured  piece 
is  found  to.be  smooth,  like  the  side  of  a  crystal.  This  is 
because  the  mineral  is  cut  by  many  minute  planes,  extend- 
ing parallel  to  one  another,  which  are  known  as  cleavage 
planes  (Fig.  106) ;  and  when  the  mineral  is  broken,  it 
splits  most  easily  along  these  planes.  Therefore,  al- 
though nearly  of  the  same  hardness  (quartz  being  a 
little  harder),  these  minerals  may  be  easily  told  apart.^ 

1  Excellent  observation  lessons  may  be  given  by  means  of  small  chips 
or  pebbles  from  a  stone  yard  or  stream  bed. 


224 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPBT 


Although  nearly  as  hard  as  quartz,  and  not  a  soluble 
mineral,  feldspar  is  not  nearly  so  durable.  This  is  because 
when  exposed  to  the  air  it  slowly  changes  and  gradually 
crumbles.  Really  fresh  feldspar  is  glassy  and  looks  quite 
like  quartz;  but  this  kind  of  feldspar  is  not  common 
excepting  in  recently  cooled  lavas,  in  which  there  has  not 
been  time  enough  for  change.  The  dull  opaqueness  of 
most  feldspar  is  due  to  the  beginning  of  this  change ;  and 
when  it  lias  gone  far  enough,  the  formerly  hard  mineral  is 
changed  to  a  crumbling  clay  or  kaolin,  just  as  hard  iron 
or  steel  may  rust  to  a  soft,  yellow,  clayey  mass.     Air  and 

water  cause  many  changes 
among  the  minerals,  and  hence 
they  slowly  crumble  when  ex- 
posed to  the  action  of  these 
agents.  Since  feldspar  is  so 
easily  altered,  it  is  an  element 
of  weakness  in  any  rock. 

Calcite,  another  common 
mineral,  forms  the  marbles 
and  limestones,  and  is  present 
in  small  quantities  in  many 
other  rocks.  It  illustrates 
still  other  mineral  properties. 
Like  feldspar  it  has  cleavage, 
but  this  is  much  more  distinct  in  calcite,  and  one  can 
rarely  find  a  piece  of  it  in  which  the  smooth,  shiny  cleav- 
age faces  do  not  appear.^  In  color  it  varies  greatly,  as 
do  the  minerals  described  above.  While  quartz  and  feld- 
spar cannot  be  scratched  with  a  knife,  calcite  is  easily 
cut.  Moreover,  unlike  these  two  minerals,  it  is  readily 
1  For  instance,  in  the  white  marble  used  for  monuments  in  cemeteries. 


Fig.  106. 

A  piece  of  calcite,  showing  cleav 
age  faces  and  cleavage  cracks. 


THE  babth's  crust  225 

carried  in  solution,  and  the  water  of  the  land  and  sea 
always  carries  it.^  One  may  see  the  proof  of  this  by 
visiting  a  cemetery  in  which  the  stones  have  stood  for 
some  years.  The  surface  of  the  marble  headstones  is 
often  etched  by  the  rain,  and  sometimes  the  inscriptions 
are  nearly  obscured. 

When  exposed  to  the  action  of  the  air,  calcite  does  not 
change  or  decay,  though  it  may  be  carried  off  in  solution; 
but  if  water  bearing  some  acid  comes  in  contact  with  it, 
a  chemical  change  immediately  takes  place,  and  the  calcite 
slowly  disappears.  What  happens  then  may  be  seen  on 
a  very  much  more  extensive  scale,  by  placing  a  bit  of 
calcite  or  marble  in  a  weak  solution  of  hydrochloric, 
or  other  acid,  when  an  effervescence  begins  and  carbonic 
acid  gas  escapes,  until  finally  the  calcite  is  entirely  gone. 
Because  this  mineral  is  soft,  easily  dissolved,  and  changed 
when  acids  come  in  contact  with  it,  rocks  that  are  made 
of  calcite  are  not  nearly  so  durable  as  those  made  of  quartz 
or  feldspar,  or  of  these  combined. 

There  are  numerous  other  important  minerals  in  the  rocks,  but  the 
three  above  mentioned,  together  with  the  clays,  etc.,  which  have  been 
formed  from  their  destruction,  constitute  considerably  more  than  one- 
half  of  the  earth's  crust.  The  other  more  common  rock-forming  min- 
erals are  the  micas,  some  of  which  are  colorless  and  others  black,  while 
all  are  characterized  by  a  remarkable  cleavage  ;  hornblende  and  augite, 
dark  brown,  green,  or  black  minerals;  and  some  of  the  compounds  of 
iron,  which  give  the  red  and  yellow  colors  to  soils  and  many  rocks. ^ 

1  It  is  this  which  makes  it  possible  for  animals  to  build  shells. 

2  The  study  of  these  and  a  few  other  minerals,  in  which  each  student 
is  expected  to  handle  the  specimens  and  determine  the  lustre,  cleavage 
(if  present),  fracture,  color,  hardness,  crystal  form  (if  present),  specific 
gravity,  etc. ,  will  be  of  great  disciplinary  value,  particularly  if  in  study- 
ing mineral  specimens,  some  of  the  crystalline  rocks  are  furnished  them 
to  study  and  to  identify  the  minerals  of  which  they  are  made. 

Q 


226 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Rocks  of  the  Crust :  Igneous  Rocks.  —  Rocks  like  those 
found  on  the  land  are  even  now  forming  on  the  earth's 
surface.  Whenever  a  volcano  breaks  forth  in  eruption, 
and  lava  flows  out  at  the  surface,  a  new  rock  is  being 
added  to  the  crust,  and  at  all  times  in  the  past,  similar 
lavas  have  come  from  within  the  earth,  and  upon  cooling 
have  formed  hard  rocks  (Chapter  XX).     Upon  the  flanks 

of  Vesuvius  and  other  volcanoes, 
we  find  such  lavas  which  have 
recently  cooled;  and  in  former 
times,  volcanoes  existed  and  sent 
forth  molten  rock  in  parts  of  the 
earth  which  are  no  longer  the 
seats  of  eruption. 

If  we  examine  a  solidified  lava, 
either  one  that  has  just  cooled, 
or  one  that  was  formed  long  ago, 
we  find  that  it  is  made  of  min- 
erals, though  the  mineral  grains 
may  be  so  minute  that  none  can 
be  distinguished  by  the  eye. 
(Compare  Plate  17  (diabase)  and 
Fig.  107.)  The  minerals  that  are 
most  common  in  these  beds  are 
feldspar,  quartz,  hornblende  and 
augite,  and  the  grains  vary  greatly 
in  size.  When  the  lavas  are  melted,  they  are  made  of 
elements  which  are  not  definitely  combined ;  but  as  they 
cool,  and  begin  to  form  hard  rock,  these  elements  come 
together  and  form  definite  compounds,  or  minerals,  some- 
what as  salt  crystals  are  produced  when  a  solution  of  hot 
salt  water  is  cooled.     If  the  lava  cools  very  slowly,  the 


Fig.  107. 

Section  of  diabase  (Plate  17) 
enlarged  under  the  micro- 
scope, showing  the  minerals. 


TEE  EABTH's  crust 


227 


crystals  have  time  to  grow  to  large  size  (Plate  17) ;  but  if 
the  cooling  is  rapid,  they  may  be  so  small  that  the  un- 
aided eye  cannot  distinguish  them ;  and  indeed  the  rock 
may  even  become  a  glass,  like  obsidian,  in  which  even  the 
microscope  cannot  detect  crystal  grains. 

Some  lavas  which  are  thrust  into  the  earth,  and  which 
have  not  reached  the  surface,  have  cooled  so  very  slowly 
that  the  mineral  grains  have  all  grown  to  good  size.  This 
is  the  origin  of  the  granites  (Fig.  108) ;  and  such  a  rock  is 
seen  at  the  sur-  ^ov-ca/^^ 


^^«g^^p^^»£^G'NAL     SURFA^^ 


Fig.  108. 

Diagram  to  illustrate  the  way  in  which  granite  is 
tlirust  into  the  earth  and  later  reached. 


face  only  when 
the  layers  be- 
neath which  it 
was  formerly 
buried  have 
been  removed 
by  the  agents 
that  are  always 
at  work  wear- 
ing away  the 
crust  (Chapter  XV).  Not  only  are  there  these  differ- 
ences in  texture,  but  some  lavas  are  porous,  others  dense, 
and  in  fact  some  are  so  porous,  that  like  pumice  they  will 
float  on  water.  The  pores  are  caused  by  the  expansion 
of  steam  in  the  cooling  lava,  for  all  of  these  molten  rocks 
contain  water.  There  is  also  a  difference  in  chemical  com- 
position of  the  lavas,  and  hence  in  the  kind  of  minerals 
which  grow  as  they  cool.  As  a  result  of  this,  some  rocks 
contain  quartz  and  others  have  none  of  this  mineral,  some 
have  hornblende,  some  augite,  etc.  As  these  differences 
occur  there  is  a  variation  in  thQ  color,  some  being  black 
and  some  nearly  white.     Ift  ^gcQrd/ince  with  these  differ- 


228  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

ences  the  igneous  rocks  (those  formed  from  the  cooling  of 

lava)  are  classified  and  given  different  names.^ 

Sedimentary  Rocks.  —  When  exposed  to  the  air,  lavas 
and  all  other  rocks  are  subjected  to  changes,  resulting  from 
the  decay  of  some  of  the  minerals  (like  feldspar,  augite, 
and  hornblende),  the  solution  of  others,  and  the  breaking 
up  of  the  grains  by  action  of  frost,  etc.  (Chapter  XV), 
so  that  in  the  course  of  time  the  rocks  slowly  crumble. 
As  a  result  of  this,  there  are  quite  different  products  of 
the  changes  in  such  minerals  as  feldspar.  New  chemical 
compounds  are  produced,  some  being  soluble  and  others 
insoluble.  The  former  may  pass  off  in  the  water,  which 
is  always  sinking  into  the  ground,  but  the  latter  remain 
behind,  generally  in  the  form  of  fine-grained  clay,  like 
kaolin.  A  third  product  is  the  unchanged  mineral,  like 
the  grains  of  quartz.  All  of  these,  taken  by  the  water 
which  flows  over  the  land,  find  their  way  into  the  rivers, 
then  from  these  either  into  lakes  or  the  sea.  Here  the 
waves  take  the  fragments,  adding  other  substances  which 
they  themselves  rasp  from  the  coast,  and  strew  them 
over  the  bed  of  the  sea  near  the  land.  The  beds  of  rock 
thus  formed  are  called  sedimentary^  and  upon  the  land 
there  are  great  areas  of  these,  which  were  once  formed 
in  the  sea,  but  are  now  raised  above  it. 

The  soluble  substances,  like  salt,  gypsum,  or  carbonate 
of  lime,  may  in  some  rare  cases,  as  for  instance  in   salt 

1  It  is  impossible  to  introduce  into  this  book  a  more  complete  state- 
ment concerning  tliese  rocks  ;  in  fact  it  would  not  be  profitable  to  do  so 
unlesa-  the  student  were  supplied  with  specimens  for  individual  study. 
This  form  of  laboratory  work  I  would  strongly  urge.  A  more  complete 
statement  about  the  rocks  of  the  crust  is  contained  in  my  Elementary 
Geology^  where  the  teacher  will  find  the  necessary  information  concern- 
ing kinds  of  rocks,  names,  places  where  specimens  may  be  purchased,  etc. 


THE  EARTH'S  CRUST 


229 


lakes,  be  precipitated  from  a  saturated  solution,  forming 
layers  of  chemically  deposited  rock.  There  are  salt  and 
gypsum  beds  in  the  west  which  have  been  formed  in 
this  way;^  and  in  some  of  the  salt  lakes  of  the  Great 
Basin  of  the  west,  beds  of  carbonate  of  lime  are  even  now 
being  precipitated.  Or  the  carbonate  of  lime  may  be 
taken  from  the  water  by  animals  and  built  into  their 
shells,  which  later  gather 
into  layers  of  carbonate  of 
lime,  forming  limestone. 
The  Globigerina  ooze  and 
the  beds  of  coral  lime- 
stone, which  are  accumu- 
lating near  coral  reefs,  are 
instances  of  these  rocks, 
which  are  known  as  organic 
sedimentary  beds.  Coal  is 
another  illustration  of  this 
class ;  but  in  this  case  the 
beds  are  made  of  plant  frag- 
ments   which    have    taken 

substances  from  the  air,  as  well  as  from  the  water  of  the 
ground.  In  every  tree  there  are  mineral  substances,  and 
it  is  these  which  form  part  of  the  wood  and  coal  ash. 

But  by  far  the  most  important  group  of  sedimentary 
rocks  is  the  mechanical  or  fragmental^  which  are  made  of 
rock  fragments  of  all  kinds,  which  the  waves  and  currents 
have  carried  and  deposited  in  layers  or  strata  on  the  sea 
bed.  These  fragments  vary  in  size  from  the  very  finest 
clay  to  coarse  pebbles  and  even  boulders.  Sometimes, 
when  the  waves  are  very  strong,  the  latter  may  be  carried; 
1  This  is  the  origin  of  many  of  the  beds  of  rock  salt  which  are  now  mined. 


Fig.  109. 

A  pebble  bed,  a  part  of  a  beach  formed 
in  the  coal  or  Carboniferous  period, 
now  exposed  at  Cape  Breton  Island, 
Nova  Scotia. 


230 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


but  later,  when  the  sea  is  more  quiet,  only  the  smaller 
fragments  can  be  transported.  Near  the  coast  line,  where 
the  waves  are  violent,  the  deposits  are  of  coarse  pebbles 
or  sand  (Fig.  110) ;  but  in  quiet  bays,  and  far  out  to  sea, 
the  finer  clay,  which  can  no  longer  be  kept  afloat  by  the  cur- 
rents, settles  to  the  bottom  and  forms  beds  of  clay.  There- 
fore among  these 
rocks  there  is  a 
variation  in  tex- 
ture from  peb- 
ble beds  to  clay 
rocks ;  and  the 
members  of  this 
group  are  named 
upon  this  basis, 
the  pebble  rocks 
being  called  con- 
glomerates (Fig. 
109),  the  sand 
beds  sandstones, 
and  the  clay  lay- 
ers either  clai/- 
stones  or  shales. 
Like  the  lavas, 

the  f ragmen tal  rocks  are  unconsolidated  when  first  formed ; 
but  as  they  are  not  hot  they  do  not  become  solid  by  cool- 
ing. When  found  upon  land  these  strata  are  generally 
in  the  form  of  hard  beds ;  and  by  examination  it  will  be 
found  that  the  grains  of  which  they  are  composed  are  held 
together  by  a  cement,  somewhat  as  we  may  cause  sand 
grains  to  cling  together  by  means  of  mucilage.  The 
cement  of  these  rocks  is  deposited  fro^i  solution  by  the 


Fig.  110. 
A  sand  beach,  pebbly  at  one  end,  Cape  Ann,  Mass. 


THE  EARTH'S  CRUST 


231 


water  which  is  percolating  between  the  grains.  The  most 
common  rock  cements  are  silica,  carbonate  of  lime,  or  some 
salt  of  iron,  —  substances  which  are  being  carried  in  solu- 
tion by  most  of  the  water  which  is  flowing  over  the  sur- 
face of  the  earth,  and  sinking  into  the  ground.  Sometimes 
the  cementing  has  only  gone  far  enough  to  cause  the 
grains  to  adhere  very  slightly,  but  more  often  a  hard  and 
very  dense  rock  is  formed  by  means  of  this  cement.     This 


Fig.  111. 
A  specimen  of  coquina. 

process  of  cementing  may  often  be  seen  in  gravel  beds  and 
on  shell  beaches,  like  those  of  Florida,  where  the  shell 
fragments  which  are  thrown  up  by  the  waves,  soon  become 
transformed  into  a  rock  called  coquina  (Fig.  Ill),  which 
is  used  for  building  houses  in  these  regions.  Between  the 
clay  or  sand  bed  and  the  solid  rock  there  is  every  gradation. 

Metamorphic  Rocks.  —  Igneous  rocks  are  made  of  minerals  which 
are  fine  crystals,  and  always  have  a  crystalline  structure,  though 


232  FIRST  BOOK  OF  PHYSICAL   GEOGRAVnY 

sometimes  not  possessing  a  perfect  crystal  outline.  The  chemically 
deposited  sedimentary  rocks  are  also  frequently  crystalline ;  but  the 
sedimentary  beds  proper,  when  first  formed,  are  made  up  oi  fragments 
of  minerals,  and  are  not  crystalline.  However,  the  rocks  of  the  crust 
of  the  earth  are  subjected  to  many  changes.  The  water  passing 
through  them  alters  them  so  that  beds  of  shell  may  become  crystal- 
line calcite ;  or  the  heat  of  a  lava  mass  passing  through  the  rocks,  or 
the  heat  which  exists  in  the  earth,  may  also  cause  change.  Besides 
this,  among  mountains  the  strata  are  often  folded,  and  sometimes  even 
crumpled  (Fig.  113),  as  we  might  crumple  sheets  of  paper;  and  this 
also  causes  heat  and  change.  The  heat  results  from  the  friction  of 
the  rock  paiticles  as  they  glide  over  one  another  during  the  folding, 
as  we  may  warm  two  rocks  by  rubbing  them  together,  or  by  pressing 
them  against  a  grindstone  that  is  revolving. 

From  these  various  causes  a  third  great  class  of  rock  beds  is  formed, 
to  which  the  name  metamorphic  is  given.  These  resemble  the  igneous 
in  being  crystalline,  and  some  of  them  look  quite  like  granite.  Here 
however,  the  minerals  have  been  formed  not  by  melting,  but  by  some 
change,  or  metamorphism,  in  which  heat  and  heated  water  have 
caused  minerals  to  change  in  kind  and  form.  The  limestone  becomes 
a  marble,  the  clay  rock  2^  slate,  and  perhaps  even  a  schist;  and  meta- 
morphism may  produce  a  coarse-grained  granitic  rock,  called  gneiss 
(Plate  17).  These  metamorphic  beds  are  nmch  more  common  in 
mountainous  regions  than  elsewhere,  because  here  the  cause  for 
heat  has  been  more  pronounced.  They  exist  over  great  areas  in 
Canada,  New  England,  the  highlands  of  New  Jersey,  etc. 

Position  of  the  Rocks.  —  Lavas  may  exist  in  any  position, 
but  they  are  commonly  found  either  in  nearly  horizontal 
beds  or  in  steeply  inclined  sheets  (Figs.  108  and  112). 
The  reason  for  this  is  evident;  for  they  come  from  deep 
within  the  earth,  and  cut  through  the  rocks  nearly  verti- 
cally in  reaching  toward  the  surface,  where  if  they  come 
to  the  air,  they  flow  out  over  the  land  in  nearly  horizontal 
sheets,  as  any  pasty  liquid  would.  These  may  later  be 
buried  beneath  otlier  rocks,  or  the  wearing  away  of  the 
crust  may  reach  down  to  a  buried  lava  mass.     The  sheets 


THE  EARTH'S   CRUST 


233 


^^m 

^ 

'=^-^^^^-^^^=^^. 

i^^^^.Ei:^ii^3£^^ 

—  — . 

Fig.  112. 
Diagram  showing  a  volcano  in  cross  section. 


that  cut  through  the  strata  are  called  dikes  (Figs.  108  and 

112);  and  in  every  eruption  of  Kilauea,  in  the  Hawaiian 

Islands, dikes  are  formed 

on  the  flanks  of  the  vol-  ^°^^^^° 

cano,  as  the  lava  wells 

out  from  a  fissure  and 

flows  down  the  mountain 

sides  (Chapter  XX). 

In  the  sedimentary 
rocks  the  original  condi- 
tion is  nearly  horizontal 
beds  of  different  kinds. 
Great  sheets  of  sand  or 
clay  are  formed  over  the 
bed  of  the  sea,  and  the  grains  settle  to  the  bottom,  form- 
ing  layers   parallel   to   it,  which  are  nearly  horizontal, 

because  this  is  the  out- 
line of  the  greater  part  of 
the  ocean  floor  (Chapter 
XII).  The  layers  vary  in 
kind  and  in  texture,  for 
as  time  elapses  there  are 
many  changes.  The  cur- 
rents vary,  the  velocity 
of  the  waves  changes, 
and  the  very  level  of  the 
land  fluctuates.  These 
changes  in  conditions 
may  cause  the  deposit  first  of  a  sheet  of  clay,  then  of 
sand,  then  another  of  clay,  then  one  of  limestone,  etc. ; 
and  these  layers  may  be  thin  seams  (or  lamince)  or  great 
beds.     These  variations  in  kind  of  rock  produce  stratifica- 


Fig.  113. 
Crumpling  of  rock. 


234  FIRST  BOOK  OF  PHYSICAL   GEOGRAPUr 

tion^  and  the  different  beds  are  called  strata  (Figs.  104, 
136,  and  139).  This  difference  of  stratification  is  one  of 
the  most  characteristic  features  of  the  sedimentary  rocks, 
which  in  consequence  are  often  called  stratified.  When 
raised  into  a  dry-land  condition,  these  beds  are  most 
commonly  so  uplifted  that  they  still  remain  nearly  hori- 
zontal, as  in  the  great  Mississippi  valley  between  the 
Appalachian  and  Rocky  Mountains ;  but  sometimes  they 
are  folded,  as  in  the  case  of  these  mountains  (Chap.  XIX). 

Metamorphic  rocks  also  vary  in  position,  for  very  often  they  are 
altered  beds  which  were  originally  stratified  into  layers  of  different  tex- 
ture and  composition  ;  but  they  are  rarely  horizontal,  for  they  have  been 
metamorphosed  as  the  result  of  changes  in  which  folding  has  been  of 
importance.  Indeed,  the  nietamorpliic  rocks  are  usually  most  com- 
plexly bent  and  twisted  (Fig.  113) ;  for  when,  by  the  movement  of 
the  earth's  crust,  these  contortions  of  the  rocks  occur,  they  are  so 
folded  and  changed  that  they  are  no  longer  sedimentary  or  igneous, 
but  become  members  of  the  metamorphic  group. 

Movements  of  the  Crust.  —  The  study  of  the  rocks  proves 
that  the  earth's  crust  has  been  in  movement  in  the  past. 
On  the  land,  even  on  the  tops  of  mountains,  there'  are 
strata  which  are  the  same  in  kind  as  those  now  gathering 
on  the  sea  floor;  and  in  them  are  found  entombed  the  shells 
or  skeletons  of  animals  that  once  lived  in  the  sea.  Hence 
these  rocks  tell  us  that  the  land  where  they  now  exist  was 
once  a  sea  bed,  and  that  then  there  came  an  uplift,  as  the 
result  of  which  they  have  been  raised  perhaps  more  than 
10,000  feet  above  sea  level.  Sometimes  this  uplift  has 
been  such  as  to  leave  the  rocks  still  in  horizontal  sheets  ; 
but  in  other  places  the  strata  have  been  folded  and  broken, 
especially  among  mountains  (Chap.  XIX). 

Not  only  has  the  crust  of  the  earth  been  in  movement 


THt:  EARTH'S  CRUST  235 

in  the  past,  but  it  is  even  now  changing  position.  During 
the  earthquake  shocks  in  1835  the  land  on  the  coast  of 
Chile  was  lifted  four  or  five  feet,  and  during  many  earth- 
quakes similar  movements  have  occurred  in  other  places. 
The  shores  of  Baffin  Land  have  risen  so  recently  that 
pebble  beaches  formed  by  the  waves,  and  now  lifted  above 
the  sea  level,  have  not  been  exposed  to  the  air  long  enough 
to  have  been  covered  with  moss  and  lichen  growth.  There 
is  historical  proof  of  change  in  level  (rising)  of  the  coast 
of  Hudson's  Bay;  and  on  the  shores  of  New  England  and 


Fig.  114. 
A  section  of  horizontal  rocks  sliowing  three  fault  planes. 

New  Jersey,  tree  trunks  now  standing  below  sea  level 
show  a  recent  sinking.  On  the  coast  of  Sweden  part  of 
the  land  is  slowly  rising  and  part  sinking,  as  has  been 
proved  by  careful  measurements  made  under  the  super- 
vision of  the  government.  Scores  of  similar  instances 
might  be  introduced  to  show  that  the  crust  is  rising  here 
and  sinking  there ;  and  from  equally  conclusive  evidence 
geologists  have  proved  that  in  the  past,  these  changes, 
occupying  long  periods  of  time,  have  produced  not  only 
the  mountains  of  the  land,  but  even  the  continents. 

When  the  rocks  thus  moved  are  disturbed  very  much 
from  their  horizontal  position,  they  either  bend  or  break. 
When  breaking,  they  may  move  only  a  few  inches,  or  per- 
haps thousands  of  feet,  along  the  plane  of  breaking,  ot  fault 


236 


FIRST  BOOK  OF  PHYSICAL   GEOGIiAPBY 


plane  (Figs.  114  and  115).  Faults  are  very  common 
among  mountains,  and  in  fact  some  mountainous  eleva- 
tions are  caused  by  them,  the  broken  blocks  being  raised 

and  tilted.  In  folding  there 
may  be  an  up  or  a  down  fold 
(Fig.  116),  the  former  being 
called  an  antlcUne,  the  latter 
a  syncline ;  and  while  these 
are  sometimes  very  symmet- 
rical, they  are  often  irregular, 
and  sometimes  they  are  even 
in  the  form  of  overturned 
folds.  In  fact,  the  folding 
of  rocks  may  proceed  so  far, 
that  as  among  the  metamor- 
phic  series,  the  beds  are  act- 
ually crumpled  (Fig.  113). 
In  an  anticline  the  rocks  dip 
in  two  directions  away  from 
a  central  axis;  but  there  is  a  fold,  the  monocline  (Fig. 
117),  in  which  the  strata  dip  in  only  one  direction. 

Age  of  the  Earth.  —  Various  attempts  have  been  made 
to  state  the  age  of  the  earth  in  years ;  but  all  have  been 

OS.    O.A. 


Fig.  115. 

Photograph  of  a  small  fault  near 
Sydney,  Cape  Breton. 


Fig.  116. 
Section  of  folded  rocks  showing  anticlines  (A),  synclines  (S),  unsymmetrical 
anticlines  and   synclines  (U.A.  and  U.S.)   and  overturned  anticlines  and 
synclines  (O.A.  and  O.S.)- 

far  from  the  truth,  because  it  is  impossible  to  find  any 
means  of  telling  how  long  it  takes  for  Nature  to  perform 


THE  earth's  CBUST  237 

her  tasks  in  changing  the  earth's  surface.  In  some  places 
there  are  30,000  or  40,000  feet  of  sedimentary  strata, 
one  layer  upon  another,  that  have  been  deposited  in  the 
sea  during  some  past  time ;  and  we  know  that  the  action 
of  the  agents  of  the  sea  would  require  many  scores  of 
thousands  of  years  to  form  these  miles  of  rock. 

Some  volcanoes  have  been  watched  for  one  or  two  thou- 
sand years,  and  they  have  not  grown  much  in  size;  ye't  in 
previous  times  they  have  been  built  to  very  nearly  their 
present  great  height,  which  in  some  cases  is  a  mile  or  two 
above  the  base.     Not  only  this,  but  in  some  parts  of  the 


VT^ 

T^^^^ 

"r-^ 

-~L.~1. 

=  r_ 

----^^^^^■■^sr-';;^,,^ 

JTVPZ 

7^^^:^^""^o^>-^, 

?t 

^  — 

'  1  1  1  1 
I'll, 

'I'll 

■^^^^§fel&5 

—  - 

.-—  — 

.~-—^rs 

&^ft 

•;/.'• 

^ii-^i-i:?^ 

~^\"\\ 

^ri^ 

~    -T-       -    ^^"^ 

§ 

~ 

f^ 

Fig.  117. 
A  monocline. 

earth  great  volcanoes  have  been  built,  then  have  become 
extinct,  and  then  slowly  been  destroyed,  until  now  only  a 
few  remnants  of  the  lofty  pile  of  lava  remain  to  tell  the 
story.  Such  great  changes  require  much  time;  and  so 
also  does  the  formation  of  the  deep  river  valleys,  like  the 
Colorado,  which  in  a  lifetime  do  not  appear  to  change, 
yet  which  have  been  cut  out  of  the  rocks  by  the  slow 
action  of  the  river  water. 

The  difficulty  in  attempting  any  estimate  of  the  age  of 
the  earth  comes  from  the  fact  that  the  changes  are  very 
slow,  and  the  time  since  they  began  to  operate  very  great, 
while  our  lifetime  is  short.  Compared  with  the  time 
which  has  elapsed  since  the  beginning  of  the  geological 
history,  a  human  life  is  but  a  fraction  of  a  second.     One 


238  FIEST  BOOK  OF  PHYSICAL   GEOGRAPHY 

thing  is  noteworthy:  all  who  have  tried  to  estimate  the 
age  of  the  earth  during  recent  years  have  placed  it  as 
millions  of  years^  and  some,  hundreds  of  millions. 

Geological  Ages.  —  While  we  cannot  tell  the  age  of  the 
earth  in  years^  we  have  been  able  to  work  out  its  general 
history,  and  to  divide  this  into  stages  or  periods^  just  as 
we  divide  the  early  history  of  man  into  stages  (the  paleo- 
lithic and  neolithic),  before  he  began  to  leave  written 
records,  which  can  be  used  to  tell  the  actual  time.  These 
geological  stages  have  been  made  out  from  the  records  left 
in  the  rocks  by  the  animal  life  of  the  past.  Slowly, 
throughout  all  past  time,  animals  and  plants  have  been 
changing,  being  first  of  simple  and  lowly  forms,  and 
gradually  changing  as  higher  groups  appeared.  For 
instance,  at  first  there  were  no  true  fishes,  reptiles,  birds, 
or  mammals ;  then  fishes  appeared,  and  then  reptiles,  then 
birds,  then  mammals,  and  finally,  highest  of  all,  man  him- 
self. Careful  studies  have  revealed  this  history  of  change, 
and  we  are  now  able  to  dividie  the  earth  history  into  stages 
or  ages^  and  say  that  certain  rocks,  in  which  are  found 
animal  and  plant  remains  of  certain  kinds,  belong  to  an 
earlier  or  later  period  than  other  beds  in  which  different 
organic  remains,  ov  fossils^  occur. 

To  these  divisions  of  the  history  certain  names  have 
been  given,  and  we  have  a  kind  of  rough  chronology,  or 
time  scale.  In  the  table,  the  most  ancient  periods  are 
placed  at  the  bottom.  These  are  merely  the  names  for 
the  larger  divisions;  but  geologists  have  carried  the  chro- 
nology much  further,  and  there  are  many  names  represent- 
ing subdivisions  of  these  larger  groups.^ 

1  A  more  complete  statement  of  the  basis  for  this  chronology  will  be 
found  in  most  geologies. 


TEE  earth's  CTtUST  289 

TABLE   OF   GEOLOGICAL  AGES 


CENOZOIC 
TIME. 

Age  Of 
Mammals. 

Pleistocene 

or 
Quaternary. 

Man  assumes  importance,  particularly 
in  the  upper  part.  In  the  first  half  the 
Glacial  Period  prevailed. 

Neocene. 

■B 

Mammals  develop  in  remarkable  vari- 
ety, and  to  great  size,  while  reptiles 
diminish. 

Eocene, 

MESOZOIC 
TIME. 

Age  of  Reptiles. 

Cretaceous. 

Birds  begin  to  become  important,  rep- 
tiles continue,  and  higher  mammals  be- 
gin. Land  plants  and  insects  of  high 
types. 

Jurassic. 

Reptiles  and  amphibia  continue  to  be 
predominant. 

Triassic. 

Amphibia  and  reptiles  develop  remark- 
ably.    Mammals  of  low  forms  appear. 

PALEOZOIC 
TIME. 

The  age  of 
Invertebrates. 

Carboniferous. 

Land  plants  assume  great  importance. 

Devonian. 

Fishes  begin  to  be  abundant. 

Silurian. 

Invertebrates  prevail,  i 

Cambrian. 

No  forms  higher  than  invertebrates. 

In  part 

AZOIC  TIME. 

No  fossils  known. 

Archean.^ 

Mostly  metamorphic  rocks,  perhaps  in 
part  the  original  crust  of  the  earth. 

1  Invertebrates  of  course  continue  down  to  the  very  present ;  but  until 
the  Devonian  they  were  the  most  important  group.  The  same  is  true  of 
fishes,  which  begin  to  be  abundant  in  the  Devonian,  but  continue  down  to 
the  present. 

2  From  this  group,  some  of  the  upper  beds  have  been  given  a  new 
name,  the  Algonkian,  occurring  just  below  the  Cambrian. 


CHAPTER   XV 

THE   WEARING   A"WAY   OF   THE   LAND 

Entrance  of  Water  into  the  Earth.  —  When  rain  falls 
upon  the  surface,  a  portion  of  it  runs  directly  away,  and 
a  part  soaks  into  the  ground.  Each  of  these  portions  is 
engaged  in  the  slow  work  of  wearing  away  the  rocks  of 
the  earth's  crust.  That  part  which  sinks  into  the  soil 
carries  with  it  some  of  the  oxygen  and  carbonic  acid  gas 
of  the  air,  and  perchance  it  also  takes  some  substances 
from  the  decaying  vegetation  at  the  surface.  When  plants 
decay,  carbonic  acid  gas  is  produced ;  and  in  the  humus 
that  is  formed,  there  are  substances,  which  when  dissolved 
by  water,  transform  it  either  to  a  weak  acid  or  an  alkali. 
Wood  ashes  are  often  used  in  making  soap  because  of 
their  alkalies,  and  these  are  among  the  materials  which 
the  water  obtains  from  the  decaying  vegetation. 

Sinking  through  the  soil,  the  water  may  encounter  a 
dense  stratum,  and  while  most  will  be  turned  aside  from 
its  downward  path,  some  sinks  gradually  into  the  rock,  for  in 
every  case  the  rocks  of  the  surface  are  crossed  by  minute 
crevices.  Some  beds  are  much  more  porous  than  others, 
and  some  parts  are  more  easily  entered  than  others. 
Therefore  there  are  paths  along  which  more  water  runs 
than  elsewhere.  This  is  why  wells  dug  in  one  place  may 
not  find  a  good  supply  of  water,  while  those  near  by  en- 
counter seams  which  furnish  a  permanent  supply.     Gen- 

240 


THE  WEARING  AWAY  OF  THE  LAND 


241 


erally  these  water-producing  seams  are  not  the  result  of 
actual  underground  streams,  but  instead,  are  places  in 
which  water  trickles  through  and  between  the  rocks  more 
readily  than  elsewhere. 

There  is  much  difference  in  the  permeability  of  the  beds 
forming  the  crust.  If  water  is  poured  upon  sand,  it  quickly 
enters  and  disappears ;  but  if 
upon  clay,  the  surface  becomes 
wet,  and  very  little  water 
passes  into  the  mass  between 
the  compact  clay  grains.  Yet 
if  we  could  examine  these  mi- 
nutely, we  would  find  that 
some  actually  did  enter.  Even 
a  dense  rock  like  granite 
allows  water  to  pass  between 
and  through  the  minerals. ^ 
Water  exists  all  through  the 
earth's  crust  as  deep  as  man 
has  explored ;  it  is  trickling 
through  the  strata,  not  only 
along  the  cracks  and  joints, 
but  also  into  the  very  heart  of  the  beds  themselves. 

In  its  underground  journey,  water  near  the  surface  re- 
mains cool ;  but  it  may  pass  near  a  lava  which  has  been 
intruded  into  the  crust,  or  it  may  settle  down  below 
the  cold  upper  crust  into  the  warmer  portions,  and  in 
each  case  its  temperature  is  increased.      Laden   as   it  is 


Fig.  118. 

Section  of  rock  (gneiss)  enlarged 
by  microscope  and  showing 
cracks  along  which  water  per- 
colates. 


1  If  a  small  piece  of  this,  or  nearly  any  other  rock,  be  carefully  dried 
for  hours  and  then  weighed,  and  afterwards  soaked  in  water  and  weighed 
again,  it  will  be  found  that  it  has  become  heavier  as  the  result  of  the 
absorbed  water. 


242 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


^n^???«5?r 


Fig.  119. 

Diagram  showing  condition  exist 

ing  in  some  hot  springs. 


with  foreign  substances  (see  above),  the  water,  even  when 
cold,  may  do   much  work  of   solution  and  change ;  but 

when  warmed  it  has  its  power 
in  this  respect  greatly  increased, 
for  warm  water  dissolves  more 
easily  than  cold. 

Return  of  Underground  Water 
to  the  Surface.  Springs. — 
After  a  certain  journey  some 
of  the  water  returns  to  the  air. 
Perhaps  it  has  gone  far  enough 
to  have  been  heated,  and  then, 
when  it  flows  out  at  the  surface, 
its  temperature  may  be  as  high 
as  the  boiling  point.  We  then 
have  a  hot  spring  (Fig,  119),  or  if  the  water  escapes  by  inter- 
mittent eruption,  a  geyser.  Such  springs  have  come  to  the 
surface  from  con- 
siderable depths, 
passing  along  a 
great  break  in 
the  earth  or  a 
fault  plane  (Fig. 
114). 

More  common- 
ly, however,  the 
water  seeps 
slowly  to  the 
surface,  and  it  is 
this  gradual  seep- 
age that  supplies  the  rivers.  Indeed,  if  the  rain  that  fell 
flowed  away  directly,  the  rivers  would  be  violent  floods  at 


Fig.  120. 

Conditions  existing  in  hillside  spring.  P,  porous  rock ; 
I,  impervious  layers.  Arrows  indicate  direction  of 
flow  of  water. 


THE  WEARING  AWAY  OF  THE  LAND 


243 


one  time,  and  then  as  soon  as  the  rain  ceased,  dry  river 
channels.  But  a  part  of  the  rain  water  is  stored  in  the 
earth,  and  gradually  turned  over  to  the  streams  after  a 
short  underground  journey.  Here  and  there,  w^here  the 
conditions  favor,  the  water  comes  out  as  a  spring  (Fig.  120). 
There  are  many  ways  in  which  springs  may  be  caused,  but 
the  most  common  is  where  water,  passing  through  the  soil, 
or  a  porous  rock,  encounters  a  less  porous  bed,  along  the 
surface  of  which  it  flows  until  it  reaches  the  air.  Such 
springs  are  very  often  located  on  hillsides,  and  sometimes 
the  line  of  contact  of  sand  and  clay  beds  is  marked  by 
boggy  places  where  the  water  is  slowly  escaping. 

Artesian  Wells.  —  Men  sometimes  make  springs  artificially,  and 
these  are  called  artesian  wells.  Water,  encountering  a  porous  stratum 
which  dips  into  the  ground,  follows  it ;  and  if  it  is  prevented  from 
escaping  by  means  of  a  more  impervious  layer,  it  may  pass  on  down 
the   incline.     Such  a  stratum  becomes  a  water-bearing  layer,  from 


Fig.  121. 

Diagram  showing  conditions  favoring  artesian  wells  (A)  in  inclined  layers ; 

porous  (P)  and  impervious  (I) . 

which,  if  a  well  is  bored  to  it,  the  water  rises,  perhaps  reaching  the 
surface  as  an  artesian  well.  The  water  is  unable  to  escape  because 
of  the  overlying  beds  which  prevent  it  from  rising,  while  the  under- 
lying impervious  layers  prevent  it  from  sinking.  It  is  therefore 
under  the  pressure  of  the  water  above  it  in  the  inclined  water-bearing 
stratum.  That  is  to  say,  the  pressure  is  great  enough  to  force  the 
water  up  through  the  well  boring,  nearly  as  high  as  the  surface  of 
the  porous  layer  where  the  water  has  entered.  Hence  if  a  well  is 
bored  to  this  layer  from  a  level  below  that  where  the  water  has 


244 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Fig.  122. 

Conditions  favoring  artesian  wells  (A)  in  a 
syncline  with  porous  and  impervious  (I) 
layers. 


entered  the  ground,  the  water  will  not  only  reach  the  surface,  but 
will  rise  into  the  air  as  a  fountain. 

These  are  the  conditions  under  which  artesian  water  is  most  com- 
monly found,  and  there  are  thousands  of  such  wells  in  this  country, 
and  many  more  in  which  the  pressure  is  not  quite  strong  enough  to 

force  the  water  out  into  the 
^  air,  when  it  is  necessary  to 
raise  it  a  short  distance  by 
pumping.  A  much  rarer, 
but  even  more  favorable  con- 
dition, is  that  where  the  rock 
layers  are  bent  into  a  syn- 
cline ;  then  there  are  two 
heads  of  supply,  and  no  escape 
down  grade ;  for  the  fold 
forms  a  saucer-shaped  depres- 
sion, while  in  a  singly  inclined  layer  the  water  may  pass  downwards 
along  the  porous  stratum  which  is  dipping  into  the  earth.  In  artesian 
wells  the  water  may  come  to  the  surface  scores  of  miles  from  the  place 
where  it  entered.  In  eastern  New  Jersey  and  eastern  Texas  there  are 
such  wells,  the  source  of  whose  water  is  scores  of  miles  to  the  westward. 

Mineral  Springs.  —  Many  waters  reached  by  wells  or 
artesian  borings,  and  many  that  come  out  as  natural 
springs,  have  mineral  properties.  In  other  words,  they 
have  minerals  in  solution,  and  sometimes  these  are  so 
abundant  that  the  water  has  a  very  disagreeable  taste. 
On  the  land,  around  many  springs,  deposits  of  minerals 
are  being  precipitated,  because  when  coming  out  into  the 
air,  the  water  can  no  longer  hold  them  in  solution.  One 
may  find  iron  deposits  of  this  origin  around  many  iron 
springs  ;  and  surrounding  hot  springs,  where  the  water  cools 
when  it  reaches  the  air,  and  therefore  can  no  longer  hold  so 
much  mineral  in  solution,  there  are  sometimes  extensive 
beds  that  have  been  precipitated,  and  to  which  additions 
are  constantly  being  made.     For  instance,  the  hot  springs 


THE   WEARING  AWAY  OF  THE  LAND 


245 


of  the  Yellowstone  Park  are  depositing  extensive  beds  of 
carbonate  of  lime  (Fig.  123)  ;  and  from  the  water  of  the 
geysers,  in  the  same  region,  silica  is  being  precipitated 
(Chapter  XX). 

This  shows  that  in  its  journey,  underground  water  is 
doing  work  of  solution.  Laden  as  it  is  with  carbonic  acid 
gas  and  other  substances,  it  attacks  the  minerals  and  takes 
many  substances  away.  These  examples,  which  are  im- 
pressive because  they  appeal  to  the  eye,  are  in  reality  only 
extensive  cases  of  exactly 
what  all  underground 
water  is  doing.  Not  a 
drop  of  water  passes  into 
the  ground,  and  escapes, 
without  bringing  to  the 
surface  a  small  load  of 
dissolved  mineral. 

It  is  this  which  makes 
water  hard  ;  and  a  chemi- 
cal analysis  of  any  spring 
or  river  water  will  reveal 
some  iron,  limestone,  or 
gypsum,  or  all  of  these, 
and  other  substances  as  well.  It  is  this  which  supplies  the 
animals  in  the  sea  with  the  carbonate  of  lime  which  they 
need ;  and  it  is  this  which  supplies  the  cement  for  the 
grains  of  the  sedimentary  rocks.  One  of  the  most  impor- 
tant lessons  taught  by  this  solvent  action,  is  that  water 
in  a  river  is  always  carrying  something  away.  Every  year, 
each  large  stream  is  bearing  seaward  thousands  of  tons 
of  dissolved  mineral;  and  this  means  that  much  solid 
matter  is  taken   away  from   its   drainage   area.     In  th* 


Fig.  123. 


Hot  Springs,  Yellowstone  Park. 


246 


FIBST  BOOK  OF  PHYSICAL  GEOGBAPHT 


course  of  the  countless  ages  of  geological  time  this  small 
work  amounts  to  a  grand  total  of  land  destruction.^ 

Limestone  Caves.  —  As  there  is  a  difference  in  the  solu- 
bility of  minerals,  so  there  is  of  rocks,  some  of  which  con- 
tain an  abundance  of  soluble  minerals.     This  is  the  case 


Fig.  124. 

Howe's  Cave,  New  York,  showing  stalactites  on  roof.    Copyright,  1889,  by 
S.  R.  Stoddard,  Glens  Falls,  N.  Y. 

with  limestones  in  which  caverns  are  being  hollowed  out 
by  water  action.  Water,  when  sinking  into  the  rocks, 
chooses  some  natural  break  or  joint  as  the  easiest  way  of 
entering  the  earth  ;  and  slowly  it  enlarges  this  by  solution. 
Then,  perhaps  coming  to  a  more  impervious  layer,  it  passes 
along  this,  dissolving  the  limestone  as  it  goes.     At  first 

1  The  Mississippi  Eiver  annually  carries  into  the  sea  150,000,000  tons 
of  dissolved  minerals. 


THE  WEARING  AWAY  OF  THE  LAND 


247 


the  water  slowly  seeps  through  the  rock  along  the  numer- 
ous joints ;  then  as  these  become  enlarged,  and  the  pas- 
sage of  the  water  is  easier,  the  conditions  may  in  time 
change  until  there  is  formed  a  veritable  underground 
river,  flowing  in  a  great  cavern  which  the  water  has  dis- 
solved out  of  the  rock^  (Fig. 
124). 

In  a  limestone  country,  like 
Kentucky  near  the  Mammoth 
Cave,  much  of  the  water  sinks 
into  the  earth,  and  small  surface 
streams  are  rare,  because  the 
drainage  is  mostly  underground. 
The  surface  water  runs  down 
into  little  saucer-shaped  depres- 
sions, or  hollows,  where  it  cas- 
cades into  the  earth  to  take  up 
its  underground  journey,  emerg- 
ing, perhaps  after  a  journey  of 
miles,  as  a  spring  on  the  bank 
of  a  river.  Gradually  the  sur- 
face of  the  land  is  being  worn 
down,  and  in  time  these  caverns  are  partly  opened  to  the  air, 
and  may  be  entered,  as  are  the  Mammoth,  Luray,  and  many 
others.  In  time  this  would  go  so  far  that  the  underground 
river  becomes  a  surface  stream,  because  the  roof  of  the  cave 
has  disappeared;  but  before  this  open-stream  condition,  there 
might  be  a  natural  bridge  formed,  where  a  part  of  the  cave 
wall,  firmer  than  the  rest,  has  been  left  standing  as  a  bridge 
across  the  valley,  the  last  remnant  of  the  old  cavern. 


Fig.  125. 
The  Natural  Bridge,  Virginia. 


1  This  is  the  case  in  Mammoth  Cave,  where  not  only  is  there  a  river, 
but  one  in  which  fishes  live. 


248 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


The  water  also  slowly  percolates  into  the  limestone  rock  through 
many  smaller  crevices,  and  enters  the  cave  through  the  roof,  drop- 
ping from  this  to  the  floor.  On  this  journey  through  the  rock  it  has 
taken  some  of  the  carbonate  of  lime  into  solution;  but  upon  entering 
the  cave,  some  of  the  carbonic  acid  gas,  which  permitted  the  water  to 
dissolve  the  limestone,  escapes  into  the  air,  and  the  water  is  then 
miable  to  hold  all  of  the  lime  in  solution, 
and  is  therefore  forced  to  deposit  some  of  its 
mineral  load,  either  upon  the  cave  roof  or  upon 
the  floor  below.  Little  by  little  these  deposits 
grow,  forming  pendent  icicle-like  columns,  or 
stalactites,  from  the  roof  (Fig.  124),  and  smaller 
stalagmites  from  the  floor.  In  time  these  may 
unite,  forming  columns  (Fig.  126) ;  and  the 
weird  and  even  beautiful  way  in  which  these 
deposits  increase,  often  ornaments  the  caverns 
so  that  they  become  places  of  distinct  interest 
and  beauty,  as  in  the  case  of  the  remarkable 
Luray  Cave. 

Breaking  up  of  the  Rocks.  Methods 
Employed,  —  By  the  action  of  solution 
described  above,  rocks  are  being  slowly 
disintegrated ;  but  there  are  other 
actions  co-operating  to  cause  the  rocks 
to  crumble.  If  this  were  not  so,  much  of  the  earth  would 
be  in  the  condition  of  bare  rock,  instead  of  a  soil-covered 
land;  and  the  rivers  would  not  be  furnished  with  sediment 
to  transport  to  the  sea.  The  strata  that  form  the  land 
are  hard,  and  they  must  be  softened  and  divided  into 
bits  before  they  can  be  worn  away  and  carried  from  land 
to  sea.  This  process  of  decaying,  softening,  and  crum- 
bling the  rocks  is  commonly  called  weathering^  because 
it  is  due  to  the  action  of  the  weather. 

One  of  the  most  potent  agencies  of  rock  disintegration 
is  water.     This  is  at  work  changing  and  dissolving  as  it 


Fig.  126. 

Column  in  a  cavern, 
caused  by  union  of 
stalactite  and  sta- 
lagmite. 


TBE  WEARING  AWAY  OF  THE  LAND 


249 


passes  through  the  crevices  of  the  rock.  Every  bit  that 
is  taken  away  in  solution  weakens  the  mass,  for  then  the 
minerals  are  less  firmly  supported,  and  in  time  they  may 
fall  apart.  As  it  passes  along,  the  water  finds  many  min- 
erals ready  for  change,  as  iron  is  when  exposed  to  damp- 
ness. Sometimes  they  need  oxygen,  sometimes  carbonic 
acid  gas,  sometimes  water,  and  very  often  two  or  all  of 
these,  and  perhaps  other  substances  which  the  water  is 
bearing.     In   this  way   hard   minerals    are   softened   and 


Fig.  127. 
Effect  of  frost  action  on  mountain  top  in  Colorado. 

rocks  crumbled.  The  changes  that  take  place  in  the 
minerals  are  very  complex,  and  one  of  the  results  of 
these  is  that  substances  are  produced  which  the  water  can 
then  take  away  in  solution. 

In  cold  climates,  and  particularly  on  high  mountain 
tops,  and  in  the  high  temperate  and  Arctic  latitudes,  water 
in  the  rock  crevices  sometimes  freezes  at  night  and  thaws 
in  the  daytime.  When  ice  is  formed,  an  expansion  takes 
place,  and  being  confined  in  small  crevices,  the  ice  presses 


250         FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 

with  great  force  against  the  enclosing  walls,  with  the 
result  that  tiny  fragments  and  even  great  pieces  are  some- 
times pried  off.  Not  only  does  this  prying  apart  of  the 
rocks  break  them  up  directly,  but  it  opens  cavities  into 
which  water  more  readily  enters,  and  this  increases  its 
power  of  dissolving  and  changing  the  minerals.  Frost 
action  is  everywhere  at  work  where  rocks  and  soil  are 
exposed  to  the  air  in  cold  climates ;  but  below  the  depth 
of  a  few  feet  it  loses  its  importance,  because  the  changes 
of  temperature  do  not  extend  far  into  the  earth.  Therefore 
the  soil,  even  though  no  deeper  than  two  or  three  feet, 
serves  as  a  blanket  to  protect  the  rock  below. 

In  hot  countries  the  change  of  temperature  from  day  to 
night  causes  expansion  during  the  day  and  contraction  at 
night;  and  this,  which  is  necessarily  different  in  different 
parts  of  exposed  rocks,  causes  bits  to  snap  from  them.  On  a 
much  larger  scale  this  action  may  be  seen  when  a  fire  is  built 
against  a  stone,  or  when  a  brick  or  stone  building  burns.^ 

Plants  are  also  helping  to  destroy  the  strata.  In  their 
sap  they  take  mineral  substances  from  the  soil,  and  upon 
dying  they  furnish  carbonic  acid  gas,  humic  acid,  and 
other  substances  to  the  water,  which  sinks  into  the  earth 
through  the  mat  of  vegetation.  They  are  also  aiding 
mechanically  ;  for  as  they  grow,  their  roots  and  tiny  root- 
lets ramify  through  the  soil,  and  even  enter  the  rock  itself, 
which  they  pry  apart  as  they  grow.  Every  tree,  every 
blade  of  grass,  and  every  lichen  that  clings  to  the  surface 
of  a  boulder  or  ledge,  is  engaged  in  pulverizing  the  rock 
or  the  soil. 

1  Or  it  may  be  illustrated  by  placing  a  lighted  candle  beneath  a  piece 
of  window  or  bottle  glass,  or  even  a  chip  of  rock,  which  will  soon  crack 
because  of  the  unequal  expansion  in  different  parts. 


TBE  WEARING  AWAY  OF  THE  LAND  251 

Difference  in  Result.  —  There  is  a  great  difference  in  the 
power  of  this  weathering.  Some  strata  are  so  porous  that 
water  easily  enters  and  causes  the  changes  mentioned ;  but 
others  are  dense  and  difficult  to  penetrate.  Many  are  made 
of  insoluble  or  nearly  insoluble  minerals,  and  others  are 
easily  destroyed  by  solution.  Some  rocks,  neither  porous  nor 
soluble,  are  made  of  minerals  which  decay  rapidly.  Where 
a  rock  is  porous,  and  has  some  minerals  that  can  be  easily 
dissolved,  and  others  that  are  readily  changed  or  decayed, 
the  rate  of  weathering  becomes  much  more  rapid  than 
where  only  one  or  none  of  these  conditions  are  favorable. 
Hence  since  rocks  differ  in  composition,  one  sees,  side 
by  side,  layers  that  are  worn  aw^ay  rapidly  and  beds  that 
resist  the  weather. 

This  is  one  of  the  most  important  principles  of  the 
physical  geography  of  the  land ;  and  it  accounts  for  many 
of  the  hills,  mountains,  and  valleys.  For  instance,  Mt. 
Washington,  the  peaks  of  the  Adirondacks,  Pike's  Peak, 
and  many  other  elevations,  are  made  of  granitic  rocks 
which  are  more  durable  than  others  surrounding  them; 
and  while  neighboring  strata  have  been  crumbled  and 
carried  away,  they  have  resisted  and  stood  up,  ever 
becoming  higher  above  the  neighboring  country,  not  so 
much  because  they  were  lifted  there  at  first,  as  because 
they  have  remained  more  nearly  at  the  elevation  to  which 
they  were  raised.  In  the  same  way  the  ridges  of  the 
Appalachians  are  often  made  of  durable  sandstone  and 
conglomerate  strata,  while  the  valleys  are  frequently 
located  in  beds  of  more  easily  removed  shale  and  lime- 
stone ;  and  all  over  the  earth's  surface  similar  illustra- 
tions, great  and  small,  may  be  found. 

There  are  also  differences  according  to  the  conditions 


252         FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

which  surround  the  rocks.  Climate  is  one  of  the  most 
important  of  these  (Figs.  128  and  129).  A  moist  region 
furnishes  more  water  to  perform  the  work  than  does  an 
arid  climate,  and  hence  the  rocks  melt  down  less  rapidly 
in  the  latter  than  in  the  former  regions.  Moreover,  vege- 
tation is  more  luxuriant  in  a  moist  climate  than  in  an 
arid  one,  and  this  furnishes  to  the  water  the  tools  with 


Fig.  128. 

Effect  of  weathering  upon  a  hill  in  the  arid  regions,  where  the  horizontal 
strata  have  been  carved  by  rain  action  because  of  the  absence  of  protection 
by  plants. 

which  it  works  in  rock  decay.  In  a  very  cold  climate 
frost  is  active  and  weathering  rapid.  In  a  warm  country 
this  condition  is  absent ;  and  although  there  is  more  water 
action,  its  effect  is  not  equal  to  that  of  frost. 

In  such  cold  lands  as  the  Arctic  regions,  or  the  high 
mountain  peaks,  altitude  is  another  modifying  condition. 
Upon  a  mountain  top,  where  the  winds  are  violent,  and 
where  the  slope  is  so  great  that  the  rain  and  melting 
snows  run  down  the  mountain  sides  with  great  velocity, 


THE  WEARING  AWAY  OF  THE  LAND 


253 


the  tiny  bits  of  rock,  broken  off  by  weathering,  are  quickly 
removed,  and  so  the  rock  remains  bare  and  open  to  the  full 
effect  of  the  weather  (Fig.  127).  When  this  is  combined 
with  a  cold  climate,  the  rate  of  weathering  is  greatly 
increased. 

The  slope  of  the  land  is  another  feature.     This  has  just 
been  mentioned  in  speaking  of  mountains  ;  but  in  this  place. 


'^^CS^f^ 


e^T/ 


'**«r 


-^ 


Fig.  129. 

A  mountain  (a  butte)  in  western  Texas,  showing  cliff  exposed  to  weather 

in  an  arid  climate  where  vegetation  is  scanty. 

we  may  also  contrast  the  precipice  with  the  gently  sloping 
hillside.  In  the  former  case,  every  piece  that  is  loosened 
by  frost,  or  any  other  cause,  falls  from  the  cliff,  leaving  the 
face  bare  to  the  attacks  of  the  weather ;  but  on  the  more 
gently  sloping  hillsides  many  of  these  fragments  remain, 
and  soon,  by  accumulating,  form  a  soil  which  to  some 
extent  protects  the  rocks  from  the  action  of  the  weather 
(contrast  Figs.  136  and  139  with  142  and  143). 


254  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Effects  of  Weathering.  —  We  may  watch  a  cliff  or  a  rock 
for  years  without  seeing  any  notable  change ;  yet  if  one 
looks  at  its  surface  it  is  seen  to  be  rough  and  crumbly,  while 
within,  the  rock  is  fresh  and  hard.^  Many  stone  buildings 
have  crambled  at  the  surface  in  the  200  or  300  years 
since  they  were  built;  and  the  obelisk,  brought  to  the 
damp,  changeable  New  York  climate,  has  been  disinte- 
grating so  rapidly  that  it  has  been  found  necessary  to 
protect  it  from  the  weather.  Taking  into  consideration 
all  the  time  during  which  these  agents  have  been  at  work, 
not  merely  a  few  years,  but  tens  of  thousands  or  even 
hundreds  of  thousands  of  years,  the  slow  work  of  weatlier- 
ing  becomes  very  important  in  its  effect.  It  is  revolu- 
tionizing the  outline  of  the  land;  and  again  and  again 
mountains  have  been  raised  and  worn  down,  valleys  have 
been  dug  where  hills  once  existed,  and  the  face  of  the  land 
has  been  changed,  and  is  even  now  very  slowly  varying  in 
its  features.  In  this  work  of  change  the  wind  and  rivers 
have  aided,  but  weathering  has  been  of  prims  importance. 
If  weakness  exists  in  rocks,  this  delicate  tool  will  find  it: 
and  while  the  weak  part  is  rapidly  destroyed,  the  hard 
portions  stand  out  in  relief.  Therefore  one  of  the  most 
important  effects  of  weathering  is  the  sculpturing  of  the 
land. 

A  second  important  effect  is  the  supply  of  materials  to 
the  rivers  and  the  ocean.     In  a  gorge  the  rock  fragments 

1  This  may  be  best  seen  in  a  quarry  by  contrasting  the  fresh  rock,  that 
is  being  quarried,  with  the  decayed  surface.  Probably  the  outside  is  dis- 
colored, perhaps  by  an  iron  stain,  because  by  the  decay  of  the  minerals 
which  contain  iron,  this  change  becomes  visible,  while  others,  perhaps 
even  more  important,  are  not  noticed.  By  direct  observation  one  may 
see  that  these  rocks  are  crumbling ;  and  geologists  find  abundant  evidence 
that  all  have  been  and  still  are  being  disintegrated. 


THE  WEARING  AWAY  OF  THE  LAND 


255 


are  constantly  dropping  to  the  bottom  of  the  cliffs  (Fig. 
130),  where  they  either  enter  the  stream,  and  are  taken 
along  with  the  current,  or  if  more  material  is  supplied 
than  can  be  carried,  or  the  fragments  passing  down  the 
cliff  fail  to  reach  the  stream,  accumulate  at  the  base  of 
the  precipice,  forming  a  talus  deposit  (Fig.  134).     A  sea 


Fig.  130. 

Crumbling  of  rocks  on  a  mountain  side,  showing  the  sliding  down  of  the  frag- 
ments which  fall  from  the  cliffs. 

cliff  furnishes  debris  to  the  sea  in  the  same  way.  We 
can  see  the  importance  of  this,  and  hence  it  appeals  to  us ; 
but  more  important  still  is  the  slower  and  less  perceptible 
down-sliding  of  the  soil  fragments  on  the  more  gently 
sloping  land.  It  is  not  more  important  because  more 
rapid,  but  because  the  area  of  such  slopes  greatly  exceeds 
that  of  steep  cliffs.  Every  rain  is  washing  some  material 
down  the  hillsides  which  border  the  river  valleys.     This 


266 


FIBST  BOOK  OF  PHYSICAL   GEOGBAPHT 


is  why  the  streams  become  muddy  with  sediment  after 
heavy  rains  and  during  the  melting  of  the  snow.   . 

This  load  of  sediment  is  important  for  several  reasons. 
First,  it  keeps  the  weathered  material  from  accumulating 
on  the  rock  and  protecting  it  by  a  deep  blanket.  Hence 
the  removal  aids  iveathering ;  for  if  some  were  not  removed, 
the  rock  would  soon  be  covered  to  such  a  depth  that  frost 
and  plants  would  produce  no  effect,  and  even  percolating 

water  would  not  be  of 
great  importance.  It  is 
also  important  because 
it  gives  sediment  to  the 
sea,  where  it  is  built  into 
the  beds  of  rock,  which 
later,  raised  to  the  sur- 
face, form  such  notable 
elements  of  the  land. 
Then  too,  the  prepara- 
tion of  material  by 
weathering  furnishes 
streams  with  tools  with 
which  to  work  in  cutting 
their  valleys.  Water  by  itself  has  little  power  to  cut  the 
rocks ;  but  armed  with  pebbles,  sand,  and  clay,  the  stream 
rasps  at  its  bed  and  slowly  deepens  its  channel  (Ch.  XVI). 
To  man  the  most  important  effect  of  weathering  is  the 
formation  of  soil.  The  crumbling  of  the  rocks  furnishes  an 
accumulation  of  soil  fragments  into  which  plants  can 
easily  thrust  their  roots ;  and  the  decay  of  the  minerals 
furnishes  substances  which  are  needed  in  plant  growth, 
and  which,  being  soluble  in  water,  are  taken  from  the  soil 
in  the  sap.     This  soil  which  is  formed  by  the  decay  of 


Fig.  131. 

Decaying  granite,  Maryland.  Fragments 
of  rock  and  clay,  formed  by  decay,  sur- 
round blocks  not  yet  reached  by  weather- 
ing, and  hence  fresh  enough  to  be  quar- 
ried. 


THE   WEARING  AWAY  OF  THE  LAND 


267 


rocks,  and  which  is  the  most  common  one  in  the  world,i  is 
called  residual  soil.  It  is  one  from  which  many  of  the 
soluble  substances  have  been  removed,  and  is  hence  com- 
posed chiefly  of  the  ifisoluble  residue,  particularly  kaolin 
cla}^  and  silica.  In  such  a  soil  the  surface  portions  are 
very  fine-grained  clay,  grading  downward  to  fresh  rock, 


^&':^i^^i?^^^V7$>C^^ti^  DECAYED 

'i^^-if^v:'i-^-:-^t-=-^=^tl]^nt^^  rock 


FRESH 
ROCK 


Fig.  132. 
Diagram  to  show  the  conditions  in  a  region  covered  by  residual  soil. 

passing  first  through  a  zone  of  partially  decayed  rock 
fragments  in  a  clay  matrix,  and  then  to  soft  and  partially 
decayed  beds,  not  yet  pulverized  to  fragments.  Such  soils 
may  reach  100  or  200  feet  in  depth,  though  they  are  com- 
monly much  less. 

Erosion  of  the  Land.  —  The  land  is  being  destroyed  by 
other  agents,  which  grind  down  the  surface  and  remove 
the  fragments  by  erosion.  Here  and  there  glaciers  pass 
over  the  land  (Chapter  XVII),  and  as  they  slowly  move 
along,  dragging  soil  and  rock  fragments  with  them,  they 
scour  the  beds  over  which  they  pass,  grinding  them  down 
and  carrying  the  pieces  thus  wrested  to  their  ends,  where 
they  may  be  left  on  the  land  or  carried  away  in  the  streams 

1  In  northern  United  States  and  Europe  the  soil  has  been  brought  by- 
glaciers  ;  and  along  rivers  there  are  soils  which  have  been  accumulated 
by  the  water.  Some  are  also  formed  by  wind  action,  and  some  were 
once  deposited  in  the  sea,  and  have  since  been  raised  above  it. 


258 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


produced  by  the  melting  of  the  ice,  or  often,  where  the 
end  is  in  the  sea,  be  borne  away  by  icebergs.  Another 
agent  of  erosion  is  the  wind^  particularly  in  arid  countries, 
where  the  soil  is  not  held  in  place  by  abundant  plant  life 
(Fig.  133),  and  where,  because  of  the  dryness  of  the  climate, 


Fig.  133. 
Rain-sculptured  Bad  Lands  of  South  Dakota. 

the  soil  particles  do  not  cling  together.  Along  the  coast 
line  the  waves  and  tides  are  ever  at  work  destroying  the  land 
and  removing  the  fragments  (Chaps.  XIII,  XVIII).  Be- 
cause of  the  great  activity  of  the  destructive  agents  in  the 
sea,  this  is  one  of  the  most  rapidly  changing  parts  of  the  land. 
On  the  land,  water  is  also  active  in  erosion.  Every 
heavy  rain  removes  some  material  from  the  surface  and 


THE   WEABING  AWAY  OF  THE  LAND  259 

carries  it  to  the  rivers.  The  raindrops,  first  striking  a 
blow  upon  the  soil,  gather  into  tinj  rills,  and  these  form 
streams ;  and  from  the  first,  impact  of  the  raindrop,  the 
water  may  be  engaged  eitlier  in  the  removal  of  loose 
particles,  or  the  grinding  of  hard  rock.  The  erosion  by 
rain  itself  becomes  more  important  when  vegetation  is  not 
present  to  check  the  force  of  its  fall  and  to  hold  it,  pre- 
venting its  rapid  escape.  Therefore,  in  roads,  ploughed 
fields,  and  in  the  great  desert  and  semi-desert  countries, 
rain  erosion  becomes  very  noticeable  (Figs.  128  and  133). 
Gathering  into  rivers^  the  rain-water  becomes  more  con- 
centrated, has  its  power  increased,  and  the  beds  of  the 
streams  may,  therefore,  be  places  of  very  rapid  erosion 
(Chapter  XVI).  Valleys  are  cut,  and  gorges  and  even 
deep  canons  are  carved  in  the  rocks.  This  is  one  of  the 
most  potent  agents  in  the  sculpturing  of  the  land.  This 
work  of  rivers  goes  hand  in  hand  with  that  of  weather- 
ing; for  the  latter,  by  causing  the  rocks  to  crumble,  fur- 
nishes rivers  with  tools  and  materials  to  carry. 

Much  of  the  rain  that  falls  on  the  land  sinks  into  the  ground,  and 
while  there  it  not  only  dissolves  and  changes,  but  also  accomplishes 
much  mechanically,  as  an  agent  of  erosion.  The  underground  water 
lubricates  the  soil  particles  so  that  they  slide  down  the  hillsides ;  and 
this  is  one  of  the  most  important  of  the  effects  of  percolating  water. 
Gradually  the  soil  migrates  down  hill  even  if  the  slope  is  not  very  great. 

Also,  water  percolating  along  the  contact  between  a  porous  and 
a  clayey  layer,  makes  the  surface  of  the  latter  slippery,  so  that  if  the 
slope  is  steep,  the  porous  bed  may  commence  to  slide,  perhaps  form- 
ing a  tiny  landslip,  or  landslide.,  though  sometimes,  on  steep  mountain 
sides,  a  great  and  destructive  avalanche.  These  slips  of  the  land,  which 
sometimes  carry  thousands  of  tons  of  soil  and  rock,  are  often  caused 
when  the  frost  is  coming  from  the  ground  and  the  earth  is  then  made 
damp.  Or  perhaps  a  heavy  fall  of  snow  will  increase  the  weight  of 
^n  unstable  part  of  the  hiU  or  mountain  side,  causing  an  avalanche. 


260  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHT 

Destruction  of  the  Land.  —  The  combined  work  of 
weathering  and  erosion  is  called  denudation.  By  these 
agents,  slowly  operating  bul^  ever  at  work,  throughout 
the  long  periods  of  time  during  which  the  land  has  been 
exposed  to  the  air,  the  most  profound  effects  have  been 
produced.  Not  merely  has  the  land  been  sculptured  into 
its  present  outline  of  hill  and  valley,  but  great  mountains 
have  been  reduced  to  lowlands,  and  thousands  of  feet  of 
rock  have  been  removed  from  the  surface.  Denudation 
is  engaged  in  the  great  task  of  destroying  the  land  and 
transporting  to  the  sea  the  materials  thus  derived ;  and  if 
it  had  been  permitted  to  work  uninterruptedly  through- 
out all  past  time,  the  land  long  before  now  would  have 
been  reduced  to  a  nearly  level  surface. 

But  it  is  not  permitted  to  work  without  interruption. 
The  land  is  rising  here  and  falling  there ;  and  as  a  result 
of  the  contraction  of  the  heated  globe,  the  land  sur- 
face is  steadily  rising,  though  now  and  then  locally  sink- 
ing, while  the  bed  of  the  ocean  is  gradually  becoming 
deeper.  Therefore,  while  the  land  is  being  attacked 
by  denudation,  it  is  also  rising ;  and  we  may  be  certain 
that  this  uplift  has  been  more  rapid  than  the  down- 
cutting  caused  by  denudation,  otherwise  the  surface 
would  be  less  rugged  and  the  level  lower.  The  two 
are  in  conflict,  and  so  far  denudation  is  the  weaker  of  the 
combatants ;  but  as  a  result  of  the  conflict,  the  face  of  the 
land  has  been  battered  and  carved  into  the  irregularities 
of  seashore,  plain,  plateau,  hill,  valley,  and  mountain.  It 
will  be  interesting  to  look  a  little  more  closely  at  some  of 
the  methods  employed  in  this  battle,  and  at  some  of  the 
results  which  have  been  produced. 


CHAPTER   XVI 

RIVER   VALLEYS,   INCLUDING  WATERFALLS   AND 
LAKES 

Characteristics  of  River  Valleys. — Rivers  occupy  valleys, 
and  among  these  there  is  an  extremely  great  variety  of 
form.  Some  are  narrow,  some  broad,  some  deep,  and  some 
shallow.  In  every  single  case  there  is  a  certain  relation 
between  the  conditions  surrounding  the  river,  and  the 
form  of  its  valley.  In  such  an  elementary  book  as  this 
we  can  do  no  more  than  understand  some  of  the  simplest 
principles,  though  geologists  studying  a  river  valley  can 
generally  find  out  why  it  has  its  particular  form.  Some 
valleys  are  situated  in  easily  destroyed  rocks,  some  in 
durable  layers,  and  some  cross  first  one  and  then  another. 
Many  are  situated  on  plains,  others  in  mountains  and 
plateaus,  and  some  exist  in  moist,  others  in  arid,  climates. 
In  many  cases  rivers  have  occupied  their  valleys  for  a  very 
long  time  ;  but  a  few  have  existed  for  only  a  short  period. 
With  all  of  these  variations,  there  is  a  resulting  variety  in 
river-valley  form ;  but  there  is  one  characteristic  of  all 
rivers,  —  that  they  flow  in  some  kind  of  a  valley. 

Another  universal  fact  is  that  at  some  time  all  river  val- 
leys contain  water.  In  most  of  them  some  water  is  always 
present,  sometimes  a  very  small  amount,  sometimes  veri- 
table floods;  but  there  are  some  valleys  which  contain 
water  only  for  a  very  short  time  during  the  year,  and 

261 


262 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


others  which  may  remain  dry  for  many  years.  That  is  to 
say,  while  water  is  in  every  case  sometimes  present,  in  all 
cases  the  amount  varies  from  time  to  time.  The  velocity 
of  the  stream  differs  with  the  slope  of  its  bed,  and  some 
have  steep,  others  gentle  slopes ;  but  with  any  given 
grade^  the    velocity   of    the   water   also   varies   with    the 

amount  of  water.  To 
prove  this  let  any  one 
examine  a  stream  that 
flows  quietly  along  in 
ordinary  times,  and  con- 
trast this  with  the  tor- 
rent that  rushes  over 
the  same  slope  after  a 
heavy  rain,  or  when  the 
snow  is  rapidly  melting. 
In  all  rivers  this  water 
supply  comes  partly 
from  the  direct  fall  of 
rain,  and  partly  from  un- 
derground water  which 
once  fell  as  rain,  and 
,       .    ,  .         ]\'^\  ^"^-        .     ,  ,  ,.  ,   then  entered  the  earth. 

A  typical  river  valley  in  mountains  (the  high  ^  77      • 

Andes  of  Peru),  showing  gorge  cut  by  All  Vlvei'S   have    tnOU- 

rapidlydesceuding  stream,  and  also  talus  ^^^^          ^^^     ^j^e^^     ^^rv 

supplying  debris  from  the  valley  walls.  .     ^ 

in  number  and  kind. 
Perhaps  some  tributaries  are  great  rivers,  like  the  Ohio 
where  it  enters  the  Mississippi,  but  most  are  only  tiny 
rills  which  exist  during  rains.  Every  river  occupies 
a  certain  hasin  or  drainage  area,  and  the  combination  of 
all  the  streams  in  this  area  forms  the  river  system.  Here 
too,  there  is  great  variety  in  form,  size,  and  conditioa, 


BIVER    VALLEYS  263 

Two  neighboring  river  systems  are  separated  by  a  line;  or 
more  commonly  by  an  area,  known  as  a  divide  or  water 
parting.  This  may  be  a  sharp  mountain  ridge,  or  much 
more  commonly,  a  gently  rounded  hilltop,  or  even  a 
swampy  plain. 

Most  of  these  features  are  ever  changing ;  for  the  val- 
leys, valley  walls,  tributaries,  divides,  and  areas  of  the 
river  systems  are  caused  to  be  what  they  are  by  the  com- 
bined action  of  running  water  and  weathering.  That  this 
is  so,  is  proved  by  the  fact  that  all  rivers  are  at  some  time, 
and  many  are  at  all  times,  carrying  loads  of  minerals  in 
solution  and  rock  fragments  in  suspension.  A  river  is 
nothing  more  than  a  drainage  line  on  the  land,  by  which 
the  surface  water  is  passing  from  high  to  low  ground, 
generally  toward  the  sea.^  In  its  course,  because  of  the 
co-operation  of  weathering,  and  by  its  own  action,  the  river 
is  obtaining  mineral  matter  to  remove  from  the  land 
(Fig.  134).  Hence  it  also  becomes  a  carrier  of  fragments 
obtained  from  the  waste  of  the  land.  Incidentally,  be- 
cause of  these  two  facts,  the  river  is  grinding  a  valley. 
That  is  to  say,  the  water  flows  down  hill,  and  is  furnished 
with  rock  fragments ;  and  with  these  in  its  grasp  it  scours 
its  bed,  ever  deepening  it  when  this  is  possible.  There- 
fore, according  as  the  slope,  volume  of  water,  amount  of 
sediment,  kind  of  rock,  and  length  of  time  in  which  the 
work  has  proceeded,  vary  from  place  to  place,  the  form 
of  the  river  valley  also  varies.  This  point  of  river  varia- 
tion from  time  to  time  and  from  place  to  place  Avill  be 
considered  in  some  detail. 

1  In  enclosed  basins,  like  the  Great  Basin  of  the  west,  or  the  Dead  Sea, 
the  water  flows  into  an  interior  area  and  hence  not  necessarily  toward 
the  ocean. 


264 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


The  River  Work.  —  When  water  gathers  into  a  stream, 
and  courses  along  down  a  slope,  it  cuts  into  its  channel 
by  two  kinds  of  action.  It  dissolves  such  substances  as 
it  can,  and  it  scours  its  bed  by  dragging  rock  fragments 
along.  The  rate  at  which  it  will  do  this  varies  with  the 
velocity  of  the  water,  the  amount  of  sediment,  and  the 
kind  of  rock  over  which  it  flows.     Limestone  is  dissolved 


Fig.  1o5. 
A  stream  bed  in  the  Adirondacks,  showing  boulders  that  are  moved  by  the 

floods. 


with  greater  rapidity  than  granite,  and  a  soft  clay  bed 
will  be  worn  away  more  rapidly  than  a  hard  rock. 

To  do  this  work  most  rapidly  the  stream  should  have 
sediment  with  which  to  wear  away  its  channel ;  but  some 
streams  have  too  much  sediment,  in  fact  more  than  they 
can  carry  along;  and  then  some  of  the  load  must  be  de- 
posited in  the  bed  instead  of  being  able  to  cut  into  the 
rock.      The  Platte   in  Nebraska  for  instance,   although 


RIVER    VALLEYS 


265 


flowing  down  a  moderately  steep  grade,  is  not  cutting  a 
valley,  but  is  building  up  its  bed.  On  the  other  hand, 
Niagara  River, 
above  the  Falls,  has 
so  little  sediment 
that  it  is  not  able 
to  scour  its  channel. 
When  this  river 
flows  out  from  Lake 
Erie,  it  starts  as 
clear  lake  water, 
and  as  a  result  of 
this,  the  stream  be- 
low Buffalo  flows 
almost  at  the  sur- 
face of  the  plain. 

A  stream  that 
flows  over  a  gentle 
slope  is  less  able  to 
cut  into  its  channel 
than  one  that  passes 
down  a  steep  grade, 
because  it  hurls 
rock  -  bits  against 
its  bed  with  less 
force ;  and  since  the 
velocity  becomes 
greater  when  the 
volume  of  water  in- 
creases,  it  can  cut 

into  its  bed  more  rapidly  when  in  flood  than  at  other  times. 
To  appreciate  this,  one  has  but  to  watch  the  rushing  torrent 


Fig.  136. 
View  in  Enfield  gorge  near  Ithaca,  N.Y.,  show- 
ing young  stream  deflected  by  joint  planes. 
Part  of  a  circular  pot  hole  in  the  foreground. 


266  FIBST  BOOK   OF  PHYSICAL   GEOGRAPUY 

which  courses  down  a  stream  valley  in  the  spring,  and  see 
it  carrying  along  pebbles,  and  even  boulders,  which  under 
ordinary  conditions  it  would  not  be  able  to  move,  and 
which,  when  the  flood  subsides,  are  left  standing  in  the 
channel  (Fig.  135).  The  greater  part  of  the  cutting  done 
by  most  streams  during  the  year  is  accomplished  during 
the  few  days  when  the  water  flows  as  a  torrent.  For  the 
remainder  of  the  year,  though  a  small  amount  of  work 
is  done,  the  stream  loiters  and  rests  from  its  labors.  In 
a  year  there  has  been  no  perceptible  deepening  of  the 
stream  channel;  but  in  a  few  centuries  rapidly  working 
streams  will  make  changes ;  and  in  the  great  ages  of  geo- 
logical time,  vast  results  in  valley  formation  have  been 
accomplished. 

If  we  should  go  to  any  stream  valley,  where  the  water 
is  flowing  over  the  bed  rock,  we  would  find  that  in  certain 
places,  perhaps  where  the  rock  is  soft  or  much  jointed, 
the  channel  had  been  turned  aside  for  some  distance  (Fig. 
136),  or  that  the  water  had  cut  into  those  places  more 
deeply  than  elsewhere.  Perhaps  in  places  where  the  flow 
was  increased  for  any  reason,  circular  pot  holes  had  been 
dug  (Fig.  136).  These  are  carved  at  the  base  of  water- 
falls or  in  an  eddy  of  the  stream,  where  the  swirling 
waters  have  whirled  the  pebbles  about  (Fig.  139).  In 
such  river  beds  there  is  proof  that  the  stream  is  really  cut- 
ting into  its  channel ;  for  one  can  see  that  a  hole  has  been 
dug,  or  that  blocks  have  been  cut  out ;  and  one  may  also 
see  that  softer  rocks  are  more  rapidly  worn  than  harder. 

Most  valleys  have  greater  breadth  than  depth,  and  the 
width  of  the  valley  is  very  much  more  (perhaps  miles) 
than  the  width  of  the  stream  channel.  So  far  as  we  have 
gone  in  this  study,  we  have  seen  only  that  the  stream  cuts 


RIVER    VALLEYS 


267 


Fig.  137. 

Diagram  to  show  relatively  small  amount  of  rock 
actually  cut  out  by  the  stream,  compared  with  the 
width  of  the  valley  as  broadened  by  weathering. 


its  bed;  and  if  this  were  the  whole  truth, 'Q,  river  valley 
should  be  narrow,  its  width  being  about  that  of  the  stream 
itself  (Fig.  137).  Evidently  therefore,  there  are  other 
facts  to  be  un- 
derstood. One  ^'~ 
of  these  is  that 
the  river  does 
not  flow  over  a 
straight  course, 
but  meanders 
about  (Fig.138). 
This  is  true  of 
all  streams,  and 
it  is  also  true 
that  in  meander- 
ing they  change  their  position  from  time  to  time.  Com- 
monl}^  the  place  of  greatest  velocity  of  a  river  is  in  the 
middle ;  but  when  it  begins  to  swing,  the  place  of  great- 
est velocity  shifts  first  to  one  side  of  the  valley,  then  to 
the  other.  Hence  the  current  strikes  against  the  bank 
(Fig.  138),  and  in  addition  to  the  vertical  cutting  in  the 
stream  bed,  there  is  a  certain  lateral  cutting  against  the 
valley  walls.  Since  the  swing  of  the  river  changes  from 
time  to  time,  different  parts  of  the  side  are  successively 
attacked,  and  so  by  this  action  the  valley  is  slightly 
broadened  (Fig.  138). 

The  really  great  widening  of  valleys  is  that  done  by 
the  very  slow  action  of  weathering  (Fig.  134).  This  is 
always  at  work,  and  little  by  little  the  rock  crumbles, 
passes  into  the  stream  (Fig.  130),  and  is  whirled  off 
toward  the  sea.  Every  block  that  falls  from  the  cliff, 
every  mud-laden  rill  that  courses  down  the  hillside,  and 


268 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


every  bit  of  dissolved  substance  brought  to  the  surface 
by  underground  water,  represents  a  small  contribution 
to  the  grand  sum  total  of  valley  broadening.  To  appre- 
ciate this  we  must  remember  that  the  valley  has  been  de- 
veloping, not  for  a  century  only,  but  for  many  tens  or 
even  hundreds  of  thousands  of  years. 


Fig.  1C8. 

Part  of  map,  showing  meandering  river  cutting  against  bluff  on  one  side  and 
depositing  on  opposite.    Arrows  indicate  the  strongest  current. 

History  of  River  Valleys.  —  If  a  river  were  supposed  to 
begin  upon  a  new  land,  the  water  would  take  the  lowest 
course,  and  along  this  ^  would  commence  to  dig  a  valley. 
Weathering  would  of  course  immediately  begin  to  co-oper- 
ate; but  for  awhile  the  work  of  the  river  in  cutting  its 
channel  would  be  more  rapid  than  the  action  of  weathering 
in  widening  the  valley,  because  the  body  of  water  moving 

1  Called  the  consequent  course,  because  it  is  chosen  in  consequence  of 
the  topography. 


RIVER   VALLEYS 


269 


along  the  narrow  channel  is  a  more  powerful  agent  than 
the  very  slow  action  of  weathering  and  rain  wash.  Con- 
sequently the  valley  would  be  narrow  and  its  sides  steep, 
because  the  river 
carves  the  rock  with 
such  relative  rapid- 
ity (Fig.  137).  Such 
a  stream  valley 
would  be  a  young 
valley  (Figs.  134, 
136,  139,  and  145); 
and  wherever  we 
find  a  gorge  or  canon, 
we  may  be  certain 
that  it  has  not  existed 
long  enough  for 
weathering  to  widen 
it. 

A  young  river  val- 
ley may  have  a  very 
irregular  course,  for 
it  has  taken  the  low- 
est line,  following  it 
wherever  that  may 
lead ;  and  man}^  young 
streams  pursue  a  very 
roundabout  path  to 
their  mouths.  It 
may  also  have  lakes 

in  the  course,  for  there  may  have  been  depressions  in  its 
bed  which  had  to  be  filled  up  before  the  river  could  pro- 
ceed.    As  time  goes  on,  these  will  be  destroyed;  for  each 


A  young  valley  being  cut  in  the  shale  rock  of 
central  New  York. 


270 


FIRST  BOOK   OF  PHYSICAL   GEOGRAPHY 


stream  that  enters  a  lake,  brings  sediment  which  is  slowly 
filling  it;  and  at  the  same  time  the  outlet  stream  is  cut- 
ting its  valley  down,  and  therefore  lowering  the  lake 
level.  In  time  the  combination  of  these  causes  succeeds 
in  removing  the  lake,  and  hence  lakes  are  not  liable  to  be 
found  in  valleys  that  have  passed  the  youthful  stage. 
Waterfalls  also  may  exist  in  the  course  of  a  young  stream, 
where  its  path  has  led  it  down  some  steep  slope  ;^  and 
falls  may  be  developed  as  the  river  cuts  its  bed,  passing 


Fig.  140. 
Diagram  to  illustrate  base  level  (B  E)  and  profile  of  equilibrium  (P  E) . 

over  hard  or  soft  rock.     Therefore  a  gorge  valley,  with 
lakes  and  waterfalls,  is  characteristic  of  young  streams. 

After  awhile,  as  the  stream  valley  becomes  older,  and 
reaches  the  stage  of  earli/  maturity^  there  are  differences 
in  the  conditions,  and  therefore  in  the  form  of  the  valley. 
There  is  a  level  below  which  the  stream  cannot  cut.  At 
its  mouth,  where  it  enters  the  sea,  this  is  the  sea  level, 
and  it  is  commonly  called  the  base  level  of  erosion  (Fig. 
140).  The  sea  level  is  the  permanent  base  level  of  streams, 
but  temporarily  there  may  be  a  base  level  above  this.  For 
instance,  so  long  as  a  lake  exists  in  the  course  of  a  stream, 
its  channel  cannot  be  cut  below  the  temporary  base  level 
of  the  lake  surface.     While  a  stream  may  cut  its  channel 

1  As  in  the  case  of  Niagara.    See  latter  part  of  chapter. 


BIVER   VALLEYS 


271 


down  to  sea  level  near  the  sea,  it  can  never  do  this  far 
inland,  for  the  river  must  maintain  a  slope  down  which 
the  water  can  run,  and  at  the  same  time  transport  its 


Fig.  141. 
Diagram  to  illustrate  the  development  of  a  stream  valley  from  original  sur- 
face (ee'),  through  youth  to  old  age  {gg'). 

sediment  load.     This  line,  sloping  down  toward  the  sea, 
may  be  called  the  profile  of  equilibrium  (Fig.  140).^ 


Fig.  142. 
A  broad,  mature  valley,  Ithaca,  N.Y. 

When  a  stream  in  its  down-cutting  has  reached  nearly 
to  this  lowest  possible  slope,  the  profile  of  equilibrium, 

1  Called  this  because  it  is  the  profile  in  which  an  equilibrium  is  main- 
tained between  water  and  sediment  supply,  and  river  slope. 


272 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


its  power  to  cut  further  is  very  greatly  diminished,  and 
finally,  when  the  profile  is  actually  reached,  it  ceases  to 
be  able  to  cut  into  its  channel.  But  weathering  still 
continues,  and  so  while  the  valley  is  no  longer  being 
deepened,  it  begins  to  grow  broader,  and  the  gorge  or 
canon  gives  place  to  a  valley  with  rounded  sides  (Fig. 
141).  This  is  characteristic  of  the  stage  of  maturity 
(Fig.  142). 


Fig.  143. 

A  view  on  the  Delaware,  showing  the  Water  Gap  in  the  background,  where 
the  rocks  are  hard  conglomerate,  and  the  broad,  gently  sloping  valley  in 
the  foreground,  where  the  rocks  are  softer  limestones  and  slates. 


The  length  of  time  which  it  will  take  to  reach  matu- 
rity varies  greatly,  as  it  does  in  animals  and  plants:  in 
some  climates  weathering  is  slow,  in  others  rapid;  in  some 
places  the  stream  begins  high  above  sea  level,  and  there- 
fore has  more  slope,  and  more  work  to  do,  than  others 
which  begin  on  low  plains ;  and  some  work  in  hard,  others 
in  soft  rock.  Indeed,  two  neighboring  parts  of  the  same 
stream  may  show  different  stages  of  development,  one 
being   in   soft  rock,  the  other   in  hard,  and  hence  one 


RIVER   VALLEYS  273 

having  a  more  rounded  outline  than  the  other  (Fig.  143). 
Also  the  headwaters  of  a  stream  will  be  less  mature  than 
the  lower  portions,  for  these  are  higher,  carry  less  water, 
and  have  more  work  to  perform.  Moreover,  they  cannot 
be  developed /as^er  than  the  lower  portions,  for  they  must 
wait  for  these  in  order  that  they  may  have  the  necessary 
slope  down  which  to  carry  their  sediment  supply. 

There  are  other  ways  in  which  a  mature  valley  differs  from  a 
young  one.  Waterfalls  cannot  exist,  because  the  slope  is  the  easiest 
one  possible,  and  all  falls  have  therefore  been  destroyed.  Nor  can 
lakes  exist,  for  they  have  all  been  filled.  The  river  course  may  have 
become  quite  different  from  the  original,  for  in  time  rivers  adjust 
themselves,  and  often  gradually  alter  their  direction.  Also  at  first 
the  divides  may  have  been  very  indefinite  and  the  tributaries  few;i 
but  the  tributaries  increase  in  number,  and  during  maturity  the 
divides  are  so  definite,  that  all  water  falling  on  the  land  finds  a  slope 
down  which  to  flow,  and  a  valley  in  which  to  join  with  other  water 
to  ultimately  reach  the  main  stream. 

After  the  profile  of  equilibrium  is  reached,  the  operation 
of  weathering  slowly  continues  to  broaden  the  valleys  and 
lower  the  hilltops,  until  if  everything  is  favorable,  the 
country  will  be  reduced  to  the  condition  of  a  plain 
(Fig.  141).  There  can  be  no  doubt  that  there  has  been 
time  enough  for  this;  but  there  are  no  such  old  valleys; 
and  here  is  an  illustration  of  the  combat  between  the 
elevating  and  destroying  forces.  The  land  is  rising 
faster  than  denudation  can  remove  it,  and  hence  there  are 
no  really  old  lands.     This  which  has  been  stated  is  the 

1  This  is  well  illustrated  in  both  the  Red  River  valley  of  the  North,  and 
in  Florida,  which  are  young  plains.  Here  the  divides  are  great,  nearly 
undrained  swamps,  and  the  water  that  falls  scarcely  knows  which  way  to 
flow.  In  time,  as  definite  courses  are  chosen,  other  streams  develop,  and 
gradually  the  divides  become  more  sharply  defined. 


274 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


normal  or  ideal  cycle  of  change.  In  reality  rivers  are  sub- 
ject to  many  interruptions,  which  may  be  called  accidents^ 
and  these  will  now  be  considered. 


6^ 


\L 


Fig,  144. 
Diagram  to  show  young,  inner,  and  older  outer  gorge  of  Colorado  River. 

Accidents   interfering  with  Valley  Development.  —  The 
country  in  which  a  valley  is  being  formed  may  be  raised, 

and  this  gives 
to  the  stream 
new  power  to 
cut,  for  it  in- 
c  reas  es  the 
slope.  By  this 
the  stream  is 
rejuvenated  o  r 
revived^  and  the 
gorge  condition 
may  be  pro- 
longed, or  a 
gorge  may  be 
cut  in  the  centre 
of  the  mature 
valley,  as  in  the 
case  of  the  Col- 
orado River  of 
the  west  (Figs. 
144  and  145).     Here  the  river  had  cut  down  to  its  profile 


^ 

,  j 

w^  - 

'  -^  *l*^^B 

^L^ 

-.^^^^B 

.^^KtL 

■ 

Hkj '^^^^^H|^H 

i^fl 

H 

Fig.  145. 

A  view  in  the  Colorado  canon,  showing  inner  gorge 
and  outer  broad  valley. 


RIVER    VALLEYS 


275 


of  equilibrium,  and  the  valley  sides  had  wasted  back, 
forming  a  fairly 
broad  valley.  Then 
there  came  an  up- 
lift of  the  land, 
since  which  the 
Colorado  has  cut 
a  narrow  canon, 
which  is  a  deep, 
narrow  trench  in 
the  middle  of  the 
older,  broad  val- 
ley. If  such  an 
elevation  should 
occur  along  the 
coast  line,  separate 
streams  might  be 
caused  to  unite 
into  one.  For  in- 
stance, if  the  re- 
gion about  Chesa- 
peake Bay  were 
to  be  raised  to  an 
elevation  of  200 
or  300  feet,  many 
streams  now  enter- 
ing this  by  sepa- 
rate mouths  would 
be  united,  flowing 
to  the  sea  through 
a  single  trunk  stream  (Fig.  146).  Such  elevations  have 
occurred  in  the  past. 


Fig.  146. 

Map  of  Chesapeake  Bay,  to  show  (by  heavy  line)  the 
way  in  which  the  various  rivers  would  unite  into 
a  single  trunk  stream  if  the  land  were  elevated. 


276  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

If  the  reverse  movement  of  depression  takes  place,  the 
stream  loses  most  of  its  power,  because  its  slope  is 
decreased;  and  then  it  may  build  a  broad  floodplain 
(Fig.  138),  because  it  is  no  longer  able  to  carry  all  its 
sediment,  but  must  deposit  some  in  its  bed  or  to  one  side 
of  the  channel.  If  in  this  case  the  depression  is  near  the 
sea,  the  stream  valley  maj'  be  submerged  or  drowned^  and 
in  fact  this  is  the  cause  of  many  of  the  bays,  harbors,  and 
estuaries  along  the  coast  (Plate  19).  In  these  the  sea  has 
extended  up  the  valleys,  and  this  has  separated  or  dissected 
rivers  which  once  entered  the  sea  by  a  single  mouth,  but 
which  now,  as  in  the  case  of  the  Chesapeake,  enter  the 
sea  by  separate  mouths  (Fig.  146).  During  recent  geo- 
logical times  northern  Europe  and  America  have  been 
depressed,  so  that  the  sea  enters  many  of  the  valleys, 
transforming  the  coast  to  one  of  extreme  irregularity 
(Plate  19). 

Another  way  in  which  the  normal  development  of  the 
stream  valley  may  be  interfered  with  by  movements  of 
the  land,  is  when  the  surface  changes  in  level  along  a 
relatively  narrow  line.  For  instance,  a  mountain  chain 
may  be  rising  across  a  river  valley.  In  this  case  perhaps 
the  stream  will  maintain  its  course,  cutting  into  the  rock 
as  rapidly  as  the  mountain  rises.  Such  a  stream,  called 
an  antecedent  river,  because  it  existed  before  the  mountain 
grew,  would  then  cross  the  mountains  directly,  forming  a 
deep  mountain  defile  or  gorge.  Though  many  rivers  cross 
mountains,  as  in  the  case  of  the  Delaware  at  the  Water 
Gap  (Fig.  143),  the  Susquehanna,  and  others  in  the  Ap- 
palachians, it  is  very  difficult  to  prove  that  the  cause  for 
this  has  been  that  just  mentioned.  In  the  main,  such 
mountain  valleys  are  the  result  of  later  changes,  and  this 


RIVER    VALLEYS  277 

applies  to  practically  all  in  the  Appalachians ;  but  some 
believe  that  the  Green  River,  where  it  crosses  the  Uinta 
Mountains  in  Colorado,  is  an  antecedent  river. 

Much  more  commonly  the  growing  mountain  dams  the 
stream,  forming  a  lake  which  may  later  be  filled  up  by  the 
river  sediment,  after  which  a  gorge  may  be  cut  at  the  point 
of  overflow ;  which  may  be  the  same  as  the  original  course, 
or  may  be  in  a  different  direction.  No  doubt  during 
mountain  formation  the  land  often  rises  so  rapidly  that 
the  stream  is  turned  to  one  side  or  diverted^  or  perhaps 
even  forced  to  flow  in  the  opposite  direction,  having  its 
course  reversed;  and  the  mountains  may  rise  rapidly 
enough  to  divide  a  stream  into  two  parts.  In  any  event, 
the  growth  of  a  mountain  seriously  interferes  with  the 
development  of  the  valley. 

Climates  have  changed  in  the  past.  In  the  Far  West, 
places  now  arid  once  had  a  moist  climate.  Some  streams 
in  this  region  that  were  formerly  larger,  are  now  shrunken; 
and  in  some  places,  where  the  weathering  of  the  damp 
climate  was  rapid,  there  is  now  relativel}^  little  weather- 
ing, and  the  broadening  of  valleys  is  therefore  a  slow 
process,  so  that  the  typical  valley  of  the  arid  land  is 
an  angular  canon  (Fig.  145).  In  this  western  region  the 
change  in  climate  has  caused  some  valleys  to  be  entirely 
abandoned,  and  some  streams  to  be  dissected.  For  instance, 
the  rivers  now  flowing  into  the  Great  Salt  Lake  valley  once 
united  to  form  a  larger  lake,  which  overflowed  into  the 
Columbia,  and  thence  to  the  sea;  but  now  they  all  run 
down  into  the  Great  Basin,  where  they  disappear  by 
evaporation.  Hence  many  streams,  at  present  having  an 
entirely  separate  existence,  and  distinct  mouths,  were 
once  united  into  a  single  stream. 


278  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

One  of  the  most  important  accidents  to  rivers  is  that  caused  by 
the  great  glaciers  which  once  overspread  northwestern  Europe  and 
northeastern  America.  By  this  nearly  all  of  the  rivers  have  had 
their  courses  interfered  with.  Some  were  turned  out  of  their  course, 
others  were  made  to  join  different  streams,  many  have  been  obliged 
to  cut  gorges,  and  a  very  large  number  have  had  their  channels 
choked  with  glacial  deposits,  so  that  they  have  become  locally  trans- 
formed to  lakes.  The  effects  of  the  glacial  accident  will  be  better 
understood  when  we  have  studied  glaciers  (Chap.  XVII). 

There  are  other  less  important  accidents  to  which  rivers  are  sub- 
ject. Sometimes  a  lava  flow  enters  a  valley  and  dams  the  stream 
back,  forming  a  lake  and  changing  its  course,  at  times  causing  it 
to  cut  an  entirely  new  valley,  perhaps  to  join  a  new  river  system, 
thus  rejuvenating  the  river  by  giving  it  a  new  task  to  perform.  An 
avalanche  from  a  mountain  may  produce  the  same  effect,  and  the 
blowing  of  sand  into  the  form  of  sand-hills  sometimes  dams  a  river, 
forming  a  lake. 

By  one  or  all  of  these  accidents  many  rivers  are  con- 
stantly being  retarded  in  their  development;  and  hence 
the  task  of  river-valley  formation  is  not  only  naturally  a 
slow  one,  but  one  beset  by  many  obstacles.  Indeed,  vari- 
ous portions  of  a  single  river  may  suffer  different  accidents 
and  become  composite  in  form.  Again  and  again  a  river 
may  start  its  development  only  to  be  interrupted;  and 
although  there  are  times  when  the  accidents  help  the 
work  along,  on  the  average  they  give  the  stream  some- 
thing more  to  do,  and  therefore  prevent  it  from  passing 
the  stage  of  maturity  into  that  of  real  old  age. 

The  River  Course.  —  At  first  a  stream  chooses  for  its 
course  the  easiest  slope,  whatever  this  may  be.  This 
consequent  stream  course  differs  according  to  the  locatioii 
of  the  river.  Upon  a  plain  it  may  be  very  irregular,  but 
in  general  will  follow  the  direction  of  the  slope  of  the 
plain,   as  the  rivers  of  Florida  flow  outward  from  the 


BIVER    VALLEYS 


279 


higher  central  part  of  the  state,  or  as  the  streams  of  east- 
ern New  Jersey  and  Texas  flow  outward  toward  the  sea. 


Fig.  147. 

Map  of  a  mountainous  country,  showing  rivers  parallel  to  ranges,  and  other 

features  of  drainage. 

Upon  a  country  that  has  been  glaciated,  the  surface  is  so 
irregular  that  streams  are  often  obliged  to  pursue  very 
roundabout  courses 
before  entering  the 
sea.  Among  moun- 
tains, folded  as 
they  are  into  ridges 
and  valleys,  the 
larger  streams  flow 
parallel  to  the 
ridges,  with  tribu- 
taries  running 
straight  down  the 
mountain  sides, 
and  the  main 
stream  now  and 
then  turning  abruptly  from  its  valley  between  the  ridges, 


Fig.  148. 

River  drainage  on  a  plain,  showing  rectangular 

tributaries. 


280 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


and  crossing  a  ridge  to  enter  another  valley,  which  it  fol- 
lows for  awhile  (Fig.  147).  Upon  a  plain  the  tribu- 
taries, which  form  an  arborescent,  interlocking  series, 
may  at  first  enter  the  main  stream  at  right  angles,  or 
nearly  so  (Fig.  148);  but  as  the  work  proceeds,  they 
enter  it  at  a  more  and  more  acute  angle  (Fig.  149), 
for  they  eat  against  the  downstream  bank,  because  the 
current  is  turned  that  way  by  the  velocity  of  the  water 
in  the  main  stream. 

This  consequent  course  may  be  followed  for  a  time,  but 
in  many  cases  this  course  changes  as  the  stream  develops. 
If  at  first  it  was  irregular,  and  a  longer  journey  was 
undertaken  than  necessary,  the  river  slowly  straightens 
its  course  so  as  to  flow  more  directly.     Or  if  after  cutting 

through  a  plane  of  horizontal 
rocks,  the  river  finds  itself  cut- 
ting into  tilted  beds,  it  may 
find  it  necessary  to  adjust  its 
course  to  agree  with  the  tilted 
rocks.  Such  a  river,  which  is 
called  superimposed^  after  cut- 
ting through  the  horizontal  beds, 
may  find  itself  flowing  across 
the  edges  of  a  series  of  tilted 
layers,  some  hard,  others  soft, 
whereas  if  its  course  were  just  a  trifle  different,  it  could 
follow  a  single  layer  of  soft  rock,  and  therefore  have  an 
easier  task.  In  such  a  case  the  river  gradually  changes, 
until  it  becomes  more  in  accord  with  the  rock  structure, 
and  finally  becomes  adjusted  to  it. 

Once  in  a  soft  bed,  the  river  tends  to  remain  there; 
and  as  the  stream  develops,  there  are  many  changes  in 


Fig.  149. 
Map  showing  interlocking  trib- 
utaries of   rivers    draining  a 
plain. 


BIVER   VALLEYS 


281 


course,  until  finally  it  has  found  the  easiest  path.  This 
adjustment  of  river  courses  is  not  easy  to  comprehend 
without  going  more  fully  into  the  question  than  we  are 
able  to  do  here,  and  so  it  may  be  merely  stated  as  a  fact, 
that  in  regions  where  hard  and  soft  rocks  occur  together, 
the  harder  beds  stand  up  as  hills  or  ridges,  while  the  softer 
ones  are  lowlands  and  hence  valleys,  not  necessarily 
because  rivers  were  first  located  there,  but  because  in  the 
course  of  time  soft  rocks  wear  away  more  rapidly  than  hard. 


OLD   VALLEY 


P'^-^-^S^Jvr,,^ 


Fig.  150. 

Diagram  showing  the  condition  in  parts  of  the  Appalachians  where  old  moun- 
tain tops  have  been  changed  to  valleys. 


Hence  a  stream,  perhaps  originally  smaller,  located  in  a  soft  layer, 
will  develop  more  rapidly  than  another  in  a  harder  layer,  and  gradu- 
ally will  gain  upon  its  neighbor,  and  perhaps  in  the  end  entirely  rob 
it  of  its  drainage  area.  The  stream  that  is  more  favorably  situated 
cuts  rapidly,  its  tributaries  have  more  slope,  and  hence  more  power, 
and  slowly  they  push  the  divide  back  into  the  area  of  the  less  favor- 
ably situated  stream.  Thus  there  is  a  constant  but  slow  migration  of 
divides,  for  the  streams  on  one  side  are  usually  more  powerful  than 
those  on  the  other.  Hence  the  smaller  stream  may  grow  larger,  and 
the  large  river  dwindle,  until  by  slow  changes  an  adjustment  is 
reached,  with  the  larger  stream  located  in  the  more  favorable  situa- 
tion. By  such  changes  as  this,  mountain  valleys  have  been  trans- 
formed to  mountain  tops,  and  mountain  tops  to  valleys  (Figs.  150 


282 


FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 


and  151).  This  is  common  in  the  Appalachians,  where  the  rivers 
are  well  adjusted  to  the  rock  structure.  During  this  slow  change 
there  is  sometimes  a  case  where  the  divide  is  forced  so  far  back,  that 
the  headwaters  of  the  neighboring  stream  are  actually  drawn  oft'  and 
carried  into  the  robber's  territory,  and  then  one  of  the  rivers  is 
quickly  reduced  in  size,  while  the  other  is  enlarged  by  the  capture. 


SANDSTONE. 


There  is  a  constant  battle  for  territory  between  neigh- 
boring streams,  and  those  that  have  the  greatest  slope,  or 
the  softest  rock,  or  the  heaviest  rainfall,  will  be  the  most 
successful  in  the  battle.  Therefore  we  must  look  upon 
the  river  valley  as  a  thing  ever  changing,  and  the  river 

system  as  a  thing  of 
activity  and  even  of 
life,  struggling  to 
reach  a  definite  end 
of  valley  develop- 
ment. The  river 
history  is  complex, 
and  the  valley  prob- 
ably composite ;  but 
it  has  a  story  to  tell : 
it  is  not  a  dead 
thing,  formed  and 
then  left  to  remain 
ever  the  same  with- 
out change.  Look- 
ing at  the  surface 
of  the  land,  every 
one  may  read  a  part 


'y/''m'-'<:''Z'MV/^^^^''^^V//M 


Fig.  151. 

Map  and  section  of  the  Sequatchie  valley  in 
the  southern  Appalachians,  a  river  adjusted 
to  the  rock  structure,  and  flowing  in  a  bed  of 
limestone  which  is  a  part  of  an  old  mountain. 


of  the  story  told  by  the  river  valleys ;  and  by  a  more  care- 
ful study  it  is  possible  to  decipher  many  of  the  stages  of 
the  previous  history,  which  has  been  so  briefly  outlined  here. 


l'^  'ar 


'-  \ 


"^  BATTLEDORE  I. 


'^i  HOG  ISLANDS 


''-^^*_^>^VpJ''^  °"'° '■      B     R     E     T     0     y 
SOUND 


^>  ^^,  Co<jw'"«  -Oa^ 


3IISSISSIPPI  RIYEK 

FkOW  the  passes  TA  GrjAT'p  FSAIRIE 


Facing  p{(.g-t  iSS. 


'   ■         Plate  18. 
Delta  of  the  Mississippi. 


BIVER   VALLEYS  283 

River  Deltas.  —  When  the  water  of  a  river  enters  a 
quiet  lake,  or  the  sea,  its  velocity  is  checked ;  and  if  it  is 
carrying  sediment,  some  of  this  must  be  deposited  near 
the  point  of  entrance.  Hence  near  their  mouths,  rivers  are 
dumping  a  load  of  rock  fragments,  sometimes  coarse, 
sometimes  fine.  In  this  way  deltas  are  built.  They 
are  flat-topped  plains  of  alluvial  material,  extending  be- 
neath the  sea  or  lake,  to  their  end,  which  is  a  steeply 
sloping  embankment  (Fig.  152).  This  steep  seaward  face, 
which  is  entirely  submerged,  grows  outward  as  more  and 

DELTA  PLAIN 

SEA 


IHllltii.- 

>A\\\\  \  \\\V\  \\  0.\  V\\\\\\  V 


Fig.  152. 
Cross  section  of  a  delta,  showing  its  structure  in  a  diagrammatic  way. 

more  is  added ;  and  it  remains  steep  for  the  same  reason 
that  a  railway  embankment  does  when  loads  of  gravel  are 
dumped  upon  it. 

The  reason  why  the  top  of  the  delta  is  a  plain,  is  that  its 
surface  cannot  be  raised  much  above  the  level  of  the  sea. 
It  does  rise  slightly/  above  sea  levels  because  as  the  river 
flows  out  over  the  plain  which  it  has  built,  it  flows  over 
such  a  moderate  slope,  that  the  channel  is  not  able  to  hold 
all  the  water  in  flood  times.  Then  the  plain  becomes 
transformed  to  a  broad,  lake-like  expanse  of  slowly  mov- 
ing and  shallow  water,  in  which  sediment  settles,  gradu- 
ally building  the  plain  higher,  but  never  to  any  very  great 
elevation  above  the  sea  or  lake. 


284  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Because  of  the  very  levelness  of  the  delta  plain,  the 
water  of  the  stream  that  forms  it  often  flows  through  sev- 
eral mouths  or  distributaries,  which  divide  from  the  stream 
near  the  head  of  the  delta,  spreading  out  fan-shaped. 
Hence  the  delta,  instead  of  being  formed  by  one  chan- 
nel, is  often  made  by  several,  and  its  front  is  broad, 
so  that  the  form  of  the  delta  plain,  between  its  sea 
margin  and  the  two  outer  distributaries,  is  often  trian- 
gular, like  the  Greek  letter  delta  (A),  whence  its  name. 
In  the  larger  deltas  of  the  world,  streams  flow  in  very 
uncertain  courses,  so  that  a  slight  cause  is  often  sufficient 
to  make  the  stream  abandon  one  of  its  former  channels. 
Such  rivers  as  the  Yellow  of  China  change  their  course 
frequently,  flooding  farms  and  villages  and  often  destroy- 
ing much  life.  This  is  because  the  slope  is  so  slight  that 
the  river  deposits  sediment,  and  builds  its  bed  higher,  so 
that  in  time  it  abandons  its  old  course  to  find  a  new  and 
lower  channel. 

Deltas  are  not  always,  nor  in  fact  usually,  found  at 
stream  mouths.  They  are  much  more  common  in  lakes 
than  in  the  sea,  mainly  because  lakes  are  shallow,  so  that 
less  sediment  is  needed  to  raise  the  bed  to  the  surface  of 
the  water.  The  fact  that  the  depth  is  great  is  one  of  the 
reasons  why  deltas  are  absent  from  most  river  mouths  on 
the  seashore ;  but  one  may  be  certain  that  any  stream 
which  carries  sediment  would  in  time  build  a  delta,  unless 
there  were  some  other  cause  which  prevented;  for  no  matter 
how  slow  the  accumulation  might  be,  year  by  year  it  would 
rise  nearer  the  surface,  until  finally  it  reached  sea  level. 

Among  these  interfering  causes  are  the  presence  of 
strong  waves,  tidal  currents,  or  other  movements  of  the  sea, 
which  take  the  sediment  as  fast  as  it  comes,  and  distribute 


BIVER   VALLEYS  285 

it  far  and  wide.  This  is  one  reason  why,  next  to  hikes, 
enclosed  and  nearly  tideless  seas,  such  as  the  Mediter- 
ranean, are  the  places  where  deltas  abound.  Of  course  a 
river  with  much  sediment  will  ordinarily  build  a  delta 
faster  than  one  carrying  little  or  none ;  but  there  are 
cases  of  rivers  heavily  laden  with  sediment  and  entering 
enclosed  bays,  yet  not  constructing  deltas.     Such  cases 

1 


Fig.  153. 
Alluvial  fans  in  the  arid  lands  of  the  west. 

are  those  in  which  the  bed  of  the  sea  is  smking^  or  has 
recently  sunk  so  rapidly,  that  the  river  deposit  has  not 
been  able  to  build  up  to  the  sea  level.  For  delta  forma- 
tion the  most  favorable  conditions  are  much  sediment, 
absence  of  strong  waves  and  currents,  a  sea  not  too  deep, 
and  a  sea  bottom  either  remaining  in  the  same  position  or 
else  being  slowly  elevated.  The  absence  of  the  latter 
condition  explains  the  absence  of  deltas  on  the  coast  of 
eastern  North  America  and  western  Europe,  where  the 
land  has  recently  subsided. 


286  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Even  on  the  land,  rivers  sometimes  make  a  deposit  which  some- 
what resembles  a  delta.  Where  the  stream  comes  down  from  a  steep 
mountain  valley  upon  a  plain,  its  velocity  is  checked  almost  as  effectu- 
ally as  if  it  had  entered  the  sea ;  and  if  it  is  bearing  much  sediment, 
as  it  often  is,  some  of  this  must  be  deposited  in  the  form  of  a  fan  or 
cone-shaped  accumulation,  with  the  apex  at  the  point  where  the  stream 
emerges  from  the  mountain.  Over  this  alluvial  fan,  fan  delta,  or  cone 
delta  (Fig.  153),  the  stream  flows  by  means  of  distributaries,  con- 
stantly adding  to  its  height  and  extending  its  area.  Like  a  delta 
this  deposit  is  somewhat  triangular  in  outline ;  but  it  has  not  the 
flat  surface  nor  the  steep  front  of  the  delta,  but  has  a  gradual  slope 
from  apex  to  base. 

River  Floodplains.  —  Streams  that  are  carrying  much  sediment  are 
often  obliged  to  deposit  some  of  it  in  the  channel,  in  places  where  for 
any  reason  the  current  is  checked.  This  may  happen  when  the  floods 
subside;  or  during  ordinary  times  the  condition  may  occur  in  an 
eddy  in  the  current,  or  on  the  down-stream  side  of  a  boulder,  or  of  a 
tree  that  has  become  lodged  in  the  channel.  The  bar  may  be  very 
tiny,  or  it  may  grow  to  the  size  of  an  island,  which  divides  the  river 
and  is  covered  only  in  flood  stages.  Some  island-like  bars  are  caused 
by  the  splitting  of  the  stream,  which  for  awhile  follows  two  chan- 
nels, forming  an  island  in  the  middle ;  but  in  time  one  of  these  is 
abandoned  and  one  chosen  as  the  channel  for  all  the  water.  In 
almost  any  stream  valley  of  moderate  slope,  one  may  see  a  great 
variety  of  bars,  islands,  and  partly  closed  stream  channels,  and 
by  the  study  of  them  one  may  often  find  their  cause.  Most  rivers 
are  constantly  changing  their  position,  and  as  they  do  this,  the  form 
of  their  channels. 

Oftentimes  the  river  is  bordered  on  one  or  both  sides  by  a  plain 
which  in  time  of  flood  is  covered  by  river  water  (Fig.  138).  This 
"river  bottom,"  or  floodplain,  may  be  narrow,  or,  as  in  such  large 
rivers  as  the  Mississippi,  very  broad.  Each  time  that  the  river  floods 
overspread  the  plain,  a  tiny  layer  of  sediment  is  deposited  on  its  sur- 
face, and  gradually  it  is  built  upward.  Indeed,  the  floodplain  is  really 
made  by  the  river  floods,  and  represents  the  accumulation  of  sediment, 
where  the  river  slope  is  not  great  enough  for  the  flood  water  to  carry 
all  the  load.  Extensive  floodplains  are  more  common  in  mature  rivers, 
and  particularly  near  their  mouths  where  the  slope  is  less.     After  a 


BIVER    VALLEYS  287 

delta  is  built,  and  the  river  flows  out  over  it,  it  is  transformed  to  a 
floodplain ;  and  when  a  bay  is  filled  with  a  level  sheet  of  river  sedi- 
ment, as  Chesapeake  Bay  may  some  day  be,  this  also  becomes  a  flood- 
plain. 

Some  of  the  narrow  floodplains,  especially  those  in  mountain  val- 
leys, are  made  of  coarse  gravel ;  but  the  larger  plains  are  built  of  very 
fine-grained  clay,  and  they  constitute  some  of  the  best  farming  land 
in  the  world.  Many  of  the  meadows  on  the  sides  of  small  streams 
are  tiny,  yet  true,  floodplains.  On  the  larger  plains  the  surface  is 
nearly  level  excepting  near  the  stream  bank,  where  the  elevation  is 
slightly  greater  than  on  either  side.  The  river  channel  on  both 
sides  is  bordered  by  a  low  embankment  or  natural  levee.  This  is  the 
place  where  the  rapid  current  of  the  channel  and  the  current  in  the 
middle  of  the  floodplain  come  in  contact;  and  here  the  velocity  of 
the  water  is  less,  so  that,  more  sediment  is  deposited,  thus  building 
the  embankment.  Upon  these  are  built  the  artificial  levees  which 
men  construct  in  order  to  confine  the  river  to  its  channel  and  prevent 
it  from  flooding  the  neighboring  plain. 

Over  a  broad  floodplain  the  river  flows  with  a  curving  or  meander- 
ing course,  circling  about  in  great,  swinging  curves.  These  are  con- 
stantly but  usually  slowly  changing  in  form  and  position.  In  the 
meander  the  river  current  has  its  greatest  velocity  on  one  side  (Fig. 
138) ;  and  hence  in  places  it  is  cutting  against  the  soft  banks,  thus 
increasing  its  swing  as  it  eats  its  way  into  the  land.  This  does  not 
broaden  the  channel,  but  merely  changes  its  position,  for  as  the  water 
cuts  against  one  bank,  it  deposits  sediment  on  the  opposite  one,  where 
the  current  is  not  so  rapid.  Therefore  the  width  of  the  channel 
remains  about  the  same,  but  its  position  slowly  changes,  and  in  time 
the  river  wanders  over  all  parts  of  its  floodplain.  On  many  of  these 
level  areas  the  river  increases  the  curve  of  meander  so  that  in  passing 
down  stream  upon  a  steamer,  one  may  often  look  across  a  narrow 
neck  of  land  and  see  the  river  half  a  mile  away,  but  several  miles 
distant  as  the  boat  must  go.  Gradually  eating  against  its  banks,  the 
river  sometimes  cuts  through  them,  taking  a  shorter  course  and  aban- 
doning the  "  ox-bow  "  curve,  which  then  becomes  a  somewhat  circular 
lake  in  the  floodplain,  called  an  ox-bow  cut-off.  These  are  common 
in  streams  and  there  are  many  large  ones  on  the  Mississippi  flood- 
plain. 


288 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHT 


Waterfalls. —  Waterfalls  and  rapids  are  formed  where- 
ever  the  stream  channel  changes  its  slope  rapidly.  They 
are  convexities  in  the  river  bed,  and  between  falls  and 
rapids  there  is  no  distinct  difference,  excepting  that  in 

the  one  case  the  bed 
descends  with  nearly 
vertical  slope,  and  in 
the  other  less  steeply. 
A  fall  may  change  to 
a  rapid,  or  vice  versa. 
There  are  various 
ways  in  which  these 
conditions  may  be 
introduced  into  the 
stream  bed.  By  some 
means  the  river  may 
be  turned  out  of  its 
course  and  forced  to 
fall  over  a  precipice 
or  down  a  steep  hill- 
side. This  was  the 
case  with  Niagara, 
which  at  the  close  of 
the  Glacial  Period 
found  its  course  leading  it  to  the  edge  of  the  bluff  at  Lew- 
iston,  over  which  it  fell,  forming  the  first  Niagara,  seven 
miles  distant  from  the  present  fall.  A  lava  flow,  or  a  land- 
slide, or  the  growth  of  a  mountain  across  a  stream  bed, 
may  introduce  a  similar  steep  slope  ;  and  sometimes  rapids 
or  falls  are  caused  where  a  side  stream  brings  coarse  mate- 
rials into  the  main  river,  which,  not  being  able  to  bear 
them  away,  allows  them  to  accumulate,  forming  a  steep 


f 

« 

— 

f    '.  ,     ..  ■ 

«?■■ 

"•  ••■ 

rifmi$^    '-'■- 

Fig.  154. 

A  view  at  Ludlowville,  central  New  York,  dur- 
ing time  of  low  water.  A  hard  limestone 
bed  with  soft  shales  below  causes  a  water- 
fall. Shales  beneath  cut  out  in  the  form  of  a 
cave.    A  miniature  Niagara  in  all  features. 


RIVER    VALLEYS 


289 


slope  in  a  part  of  the  bed.     There  are  rapids  of  this  kind 
in  the  valley  of  the  Colorado  River. 

But  many  streams  develop  rapids  and  falls  as  they  pro- 
ceed in  the  construction  of  their  valleys.  If  in  cutting 
down  in  their  channels  they  encounter  a  hard  layer  with  a 
soft  one  below,  they  can  cut  the  latter  more  rapidly  than 


Fig.  155. 
A  general  view  of  Niagara,  showing  Horseshoe  Falls  in  the  distance. 


the  former,  and  hence  increase  their  slope  (Fig.  154). 
This  very  increase  of  slope  gives  them  new  power  to  dig, 
and  so  they  deepen  the  channel  more  and  more,  perhaps 
in  the  end  forming  a  very  high  fall.  Many  of  the  water- 
falls of  New  York,  and  other  regions,  have  been  formed  in 
this  way,  and  Niagara,  though  first  caused  as  above  stated, 


290  FIBST  BOOK  OF  PHYSICAL   GEOGBAPHT 

still  continues  to  exist  for  this  very  reason.     Therefore  we 
may  take  Niagara  as  a  type  of  this  kind  of  fall. 

Niagara  began  as  a  waterfall  at  the  bluff  at  Lewiston  and 
has  gradually  retreated  up  stream,  until  it  has  reached  its 
present  position,  where  it  is  a  great  fall  about  160  feet  high 
(Fig.  155),  and  7  miles  from  its  original  position,  from 
which  it  is  now  separated  by  a  gorge,  200  or  300  feet  in 
depth,  which  it  has  cut  out  of  the  rock.     The  fall  is  still 


A  peat  bog  in  the  Adirondacks  with  a  pond  enclosed  —  the  last  remnant  of  the 
former  lake.     (Copyrighted,  1888,  by  S.  R.  Stoddard,  Glens  Falls,  N.Y.) 

moving  backward  at  a  rate  which  has  been  measured. 
During  the  last  50  years  Niagara  has  moved  up  stream 
about  250  feet,  yet  it  stands  at  about  the  same  elevation. 
The  reason  why  Niagara  has  thus  retreated  is  found  in  the 
difference  in  rock  structure.  Just  beneath  the  water,  at  the 
crest  of  the  Falls,  is  a  sheet  of  hard  limestone,  beneath 
which  are  beds  of  soft  shale.  Dashing  over  the  fall,  the 
water  digs  into  the  shale  and  cuts  it  out  from  beneath  the 


RIVER    VALLEYS  291 

hard  layer  of  limestone  until  some  of  this  is  undermined, 
when  a  block  falls,  and  the  waterfall  moves  up  stream 
for  a  few  feet  (Fig.  154). 

This  process  of  undermining  continues,  and  Niagara 
remains  as  a  fall  because  it  cannot  cut  the  hard  limestone 
as  fast  as  the  shale.  The  fall  will  remain  just  so  long  as 
these  conditions  exist  in  the  stream  bed;  but  it  will  of 
course  disappear  when  the  stream  has  reached  its  lowest 
slope,  or  the  profile  of  equilibrium ;  for  it  cannot  then  cut 
one  part  faster  than  another.  There  are  thousands  of  falls 
in  this  country  where  the  cause  is  the  same  as  this ;  and 
these  are  all  in  streams  that  are  young  enough  to  be  dig- 
ging into  their  beds,  and  hence  able  to  discover  differences 
in  the  hardness  of  the  rocks.  Therefore  falls  and  gorges 
are  closely  associated. 

Lakes.  —  A  waterfall  is  a  convexity  in  the  stream  chan- 
nel;  a  lake,  a  concavity.  When  for  any  reason  a  lake 
exists,  the  basin  must  be  filled  as  high  as  the  lowest  part 
of  the  rim,  where  it  will  outflow,  provided  the  rainfall  is 
sufficient.  There  are  many  ways  in  which  such  basins 
may  be  caused.  A  surface  of  new  land,  like  a  sea  bottom 
just  elevated,  may  have  saucer-like  depressions  upon  it ;  or 
these  may  be  caused  by  the  change  of  level  of  the  land. 
For  instance,  a  mountain  developing  across  a  stream 
channel,  often  causes  a  dam,  behind  which  lakes  gather ; 
or  a  lava  flow  may  check  a  stream;  or  deposits  left  by 
glaciers  may  build  dams.  The  latter  cause  accounts  for 
most  of  the  lakes  of  the  world,  especially  those  of  north- 
western Europe  and  northeastern  America  (Chap.  XVII). 
A  lake  is  therefore  a  part  of  the  river  valley. 

Lakes  will  exist  in  the  course  of  a  stream  only  for  a 
short   time,  because    rivers   "are   the   mortal   enemies  of 


292  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

lakes."  They  are  cutting  down  the  outlets  and  filling  the 
basins  with  sediment.  Generally  they  cut  very  slowly, 
because  they  have  no  tools  with  which  to  work,  having 
been  robbed  of  most  of  their  sediment  in  passing  through 
the  quiet  lake  water,  which  acts  as  a  filter.  The  lake  will 
last  until  destroyed  by  the  combined  process  of  filling  and 
cutting  at  the  barrier,  and  the  last  stage  will  be  a  swamp, 
for  when  the  water  becomes  shallow  enough,  vegetation 
commences  to  grow,  completing  the  filling,  and  transform- 
ing the  water  to  land.  The  great  majority  of  the  swamps 
of  the  world  are  the  result  of  the  filling  of  lakes.^  When 
the  lake  is  filled,  the  streams  flow  over  the  surface  of  the 
deposits,  and  being  no  longer  robbed  of  their  sediment, 
begin  to  cut  into  the  lake  beds,  and  perhaps  cut  canons 
where  the  lakes  formerly  existed.  The  filling  of  small 
ponds  is  a  small  task,  but  the  destruction  of  such  immense 
bodies  as  the  Great  Lakes  is  a  much  more  difficult  one, 
though  still  possible  if  time  enough  is  allowed. 

Sometimes  lakes  are  caused  to  disappear  by  other  means.  For 
instance,  a  great  lake  once  existed  near  Salt  Lake  City,  the  Great  Salt 
Lake  being  the  shrunken  remnant.  This  which  once  outflowed  into 
the  Columbia,  was  destroyed  by  a  change  of  climate  from  moist  to 
dry,  so  that  the  water  evaporated  faster  than  it  was  supplied.  Also 
in  the  valley  of  the  Red  River  of  the  North  an  immense  lake  has 
recently  existed,  at  the  time  when  the  great  glacier  formed  a  dam  and 
prevented  the  river  from  flowing  northward,  canfsing  it  to  outflow 
toward  the  south  into  the  Mississippi.  This  lake  disappeared  when 
the  glacier  withdrew  far  enough  to  allow  the  water  to  take  its  natural 
northward  course. 

So  also  at  a  time  when  the  glacier  prevented  them  from  outflowing 
as  at  present,  the  Great  Lakes  have  been  higher  than  now.     For  in- 

1  There  are  other  kinds  of  swamps,  the  next  most  important  being  river 
swamps  caused  by  river  water  overflowing  its  floodplain. 


RIVER    VALLEYS  293 

stance,  once  when  the  ice  occupied  the  St.  Lawrence  valley,  the  lake 
waters  rose  higher  than  now,  and  Lake  Ontario  outflowed  through  the 
Mohawk.  Even  before  this,  when  the  Mohawk  was  also  ice-filled,  the 
lakes  flowed  through  other  channels,  once  past  Chicago,  and  once  past 
Fort  Wayne,  Indiana.  While  these  high  stages  existed,  the  lake 
waters  built  beaches,  cut  cliffs,  and  deposited  layers  of  sediment  over 
their  beds,  and  these  now  appear  on  the  land,  so  that  we  may  study 
them,  and  from  them  see  what  is  now  being  done  in  lakes.  This  is 
the  reason  for  the  extensive  wheat  plains  of  Dakota  and  Manitoba,  in 
the  valley  of  the  Red  River,  which  were  built  in  the  bed  of  a  lake ; 
and  also  of  the  elevated  beaches  which  pass  through  New  York,  near 
Lakes  Ontario  and  Erie,  and  thence  westward  into  Ohio. 

Lakes  generally  have  outlets  ;  but  sometimes,  where  the 
climate  is  dry,  they  do  not  fill  their  basins  to  the  rim. 
There  are  many  such  lakes  in  the  Great  Basin,  and  in 
other  dry  regions  of  the  world,  and  among  these  are  many 
salt  lakes.  In  time  any  lake  without  outlet  will  become 
salt,  because  all  water  that  is  flowing  over  the  rocks  bears 
some  salt.  When  it  evaporates,  the  vapor  is  nearly  pure 
water,  without  the  salt,  and  hence  the  salt  is  left  behind. 
So  day  by  day,  more  and  more  salt  Is  supplied,  and  the  water 
that  brought  it  does  not  flow  away  with  it  to  the  sea,  but 
passes  into  the  air  without  it.  Thus  little  by  little  the 
fresh-water  lakes  become  salt,  and  then  Salter  and  Salter, 
turning  to  dead  seas,  and  perhaps  becoming  so  saline  that 
some  must  be  deposited  as  rock  salt  in  the  lake.  Even  the 
Great  Lakes  would  become  saline  if  the  climate  should 
become  so  dry  that  they  could  not  rise  to  their  rims. 


CHAPTER   XVII 

GLACIERS   AND   THE   GLACIAL   PERIOD 

Valley  Glaciers.  —  In  some  parts  of  the  earth  the  snow 
remains  on  the  ground  throughout  the  year.  This  occurs 
on  high  mountain  tops  or  else  in  high  polar  latitudes, 
where  much  of  the  precipitation  is  in  the  form  of  snow, 
and  where  the  effect  of  summer  melting  is  not  sufficient  to 
remove  the  supply  of  snow.  The  line  above  which  this  re- 
mains permanently  on  the  ground  is  the  snow  line.  Among 
mountains  the  snow  line,  if  present,  is  found  in  the  upper 
portions ;  and  in  temperate  latitudes  only  the  high  valleys 
and  peaks  are  covered  with  perpetual  snow.  Here,  since 
each  summer  fails  to  remove  the  fall  of  the  preceding 
winter,  snow  gathers  year  by  year,  filling  many  valleys 
and  clothing  mountain  sides  in  a  permanent  coat.  This  is 
known  as  a  snow  fields  and  it  is  from  here  that  valley 
glaciers,  such  as  those  of  the  Alps,  have  their  origin. 

A  snow  field  on  a  high  mountain  is  elevated  into  the 
zone  of  strong  winds ;  and  therefore  much  of  the  snow 
that  falls  upon  it  is  whirled  away,  settling  at  some  lower 
level,  and  very  often  in  the  valleys  between  the  peaks. 
By  this  means  there  is  a  constant  movement  from  the 
snow  fields  into  the  valleys.  Besides  this,  the  high  peaks 
of  the  mountains  are  very  steep,  and  much  of  the  snow 
that  falls  cannot  lodge  upon  the  slopes,  but  slides  down 

294 


GLACIERS  AND   THE  GLACIAL  PERIOD 


295 


into  the  valleys.  Much  more  slides  down  in  the  form  of 
great  avalanches,  or  snow  slides,  after  the  snow  has  become 
so  deep  upon  the  slope  that  it  must  slip  off.  By  this 
means  the  snow  is  prevented  from  reaching  great  depths 
in  the  high  parts  of  the  mountains,  and  much  of  it  there- 
fore passes  down  into  the  valleys,  as  water  does  on  the 


Fig.  157. 
Snow  field  in  the  high  Alps. 


land.     Indeed,  we  may  call  the  snow  field  the  supply  region 
for  the  ice  streams  or  glaciers  which  occupy  the  valley. 

In  a  snow  field  the  material  is  true  snow,  in  the  glacier 
real  ice.  There  is  a  region  between  these  two,  near  the 
head  of  the  glacier  proper,  where  the  snow  is  changing  to 
ice,  and  this  is  called  the  nev^.  By  melting  and  freezing, 
and  by  pressure,  the  snow  becomes  compacted  into  ice, 
and  then  it  slowly  flows  down  the  valleys  as  glaciers, 
passing  down  by  a  slow  movement,  somewhat  as  wax  will 


296 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


flow  if  a  large  piece  is  placed  upon  an  inclined  surface 
and  gradually  warmed.  In  other  words,  the  glacier  ice 
behaves  like  a  viscous  body. 

Hence  supplied  from  the  ice  field,  the  valley  glacier 
slowly  passes  down  the  mountain  side,  extending  well 
beyond  the  snow  field,  just  as  a  river  flows  out  beyond 


'f^t'^ 


#. 


Fig.  158. 
An  Alpine  glacier,  showing  snow  field,  ice  stream,  and  medial  moraine. 

the  place  from  which  its  water  comes.  It  will  pass 
down  as  far  as  the  supply  exceeds  the.  melting,  and 
will  then  end ;  and  the  terminus  of  the  glacier  will  there- 
fore extend  much  further  if  the  supply  is  great,  than 
it  would  if  this  were  small.  Throughout  its  passage 
down  the  mountain  valley  the  glacier  receives  much  rock 
material.  Just  as  in  the  case  of  a  mountain  river,  so  here, 
weathering  supplies  rock  fragments.     Avalanches,  single 


GLACIERS  AND   THE  GLACIAL  PERIOD 


297 


blocks,  and  bits  whirled  by  the  wind,  are  carried  upon 
the  ice,  forming  a  moraine.  Those  piles  of  rock  frag- 
ments, dropped  mainly  from  the  valley  sides,  and  resting 
on  the  surface  of  the  glacier  near  its  margin,  are  called 
lateral  moraines.  When  two  glaciers  unite,  two  of  the  lat- 
eral moraines  may  join,  forming  a  medial  moraine  (Fig.  158). 
This  is  a  dark  band  of  rock  and  gravel  in  the  middle  of 
the  ice,  and  on  some  glaciers  there  are  several  of  these. 


Fig.  159. 
Rough  crevassed  surface  of  Muir  glacier,  Alaska. 


Although  when  subjected  to  slow  pressure  the  ice  flows 
like  a  viscous  body,  if  for  any  reason  it  is  strained,  or 
caused  to  move  rapidly,  it  may  crack,  as  we  may  break  ice 
or  wax  by  striking  it  a  blow.  Therefore  when  flowing 
over  its  bed,  since  this  is  generally  an  irregular  rock  sur- 
face, it  is  often  cracked  or  crevassed.,  perhaps  becoming 
exceedingly  rough  and  even  impassable.  Where  the  val- 
ley bottom  slopes  rapidly,  an  ice  fall  may  be  caused,  in 
which  the  ice  is  crevassed  into  an  irregular  surface,  quite 
closely  resembling  a  river  surface  tossed  about  in  a  rapid. 


298  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

Through  these  crevasses  some  of  the  surface  moraine 
falls  to  the  bottom  of  the  glacier,  and  it  is  then  dragged 
along  the  bottom,  where  it  obtains  more  material  rasped 
from  the  bed.  The  rock  fragments  in  the  bottom  of 
the  ice  form  what  is  known  as  the  ground  moraine. 
This  together  with  the  lateral  and  medial  moraines, 
journeys  slowly  forward  in  the  glacier  until  the  end  is 
reached,  where  the  ice  melts  and  flows  away  in  streams, 
while  much  of  the  ice  load  of  rock  materials  remains 
behind,  forming  a  moraine  at  the  end  of  the  glacier,  the 
terminal  moraine.  If  the  ice  front  remains  at  one  place 
for  a  long  time,  the  terminal  moraine  may  be  built  to  a 
considerable  height,  being  made  of  hills  of  gravel  and 
boulders  brought  and  dumped  by  the  ice. 

As  it  passes  over  the  rock  of  its  bed,  the  valley  glacier 
acts  like  a  powerful  sandpaper,  grooving  and  polishing  the 
surface  over  which  it  passes,  and  grinding  the  fragments 
to  a  fine  clay.  In  its  mode  of  work  it  is  unlike  a  river, 
for  it  presses  down  on  its  "bed  under  the  heavy  weight  of 
solid  ice  above,  while  water,  buoying  up  the  sediment  which 
it  carries,  makes  the  pebbles  and  sand  lighter.  The  gla- 
cier differs  also  in  the  material  which  it  carries.  Since  the 
rock  fragments  are  frozen  into  the  ice,  a  large  boulder  is 
transported  as  easily  as  a  bit  of  sand,  and  so  they  journey 
along  side  by  side ;  but  in  a  stream  the  velocity  may  be 
rapid  enough  to  carry  sand,  but  not  to  carry  pebbles. 
Hence  it  is  that  the  deposits  made  by  the  glacier  are 
composed  of  bits  of  rock  varying  from  fine  clay  to  large 
boulders,  many  of  which  are  scratched  because  they  have 
been  ground  under  the  ice  (Fig.  162).  But  the  materials 
deposited  by  rivers  are  assorted  according  to  the  size  which 
can  be  carried  with  a  given  velocity.     If  the  glacier  dis- 


GLACIERS  AND   THE  GLACIAL  PERIOD  299 

appears  from  a  valley  by  melting,  —  and  many  have  done 
so  in  the  past,  —  the  moraines  are  left  on  the  surface,  per- 
haps damming  the  streams  and  forming  lakes,  or  turning 
them  out  of  their  path  into  more  irregular  courses,  in' 
which  perhaps  they  are  obliged  to  cut  new  valleys. 

From  the  front  of  the  glacier  there  generally  emerges 
a  stream  (or  sometimes  several)  coming  from  an  ice  cave, 
out  of  which  they  flow  with  considerable  velocity,  bearing 
much  sediment,  which  usually  makes  them  milky  white  in 
color.  This  they  carry  down  their  valleys,  depositing 
some  of  it  in  the  channel,  when  the  slope  decreases,  or 
perhaps  over  floodplains,  or  in  lakes.  Therefore  not  all 
of  the  material  which  the  glacier  carries,  remains  in  the 
terminal  moraine  at  the  margin  of  the  ice. 

Valley  glaciers  move  at  a  variable  rate,  depending  upon 
the  slope  and  the  snow  supply.  Their  movement  is  ordi- 
narily only  a  few  inches  or  a  few  feet  a  day,  but  some  of 
the  valley  tongues  of  ice  on  the  Greenland  coast  move  as 
much  as  75  or  100  feet  a  day.  The  glacier  movement  is 
generally  so  slow  that  it  is  necessary  to  observe  very  care- 
fully in  order  to  detect  it.  The  movement  is  more  rapid 
in  the  centre  than  on  the  sides,  where  it  is  retarded  by 
friction,  and  for  the  same  reason  less  rapid  near  the  bottom 
than  at  the  surface.  They  flow  in  valleys  which  pre- 
viously existed,  and  probably  glaciers  have  never  carved 
their  valleys  as  rivers  have ;  but  in  some  places  they  are 
widening  and  deepening  them.  Where  they  cut  into  the 
bed  more  rapidly  than  elsewhere,  they  have  carved  out 
basins  in  the  rock,  in  which  lakes  later  accumulate  after 
the  glaciers  have  left  the  valleys. 

Mountain  valley  glaciers  exist  in  the  Caucasus  Mountains,  the 
Alps,  and  in  the  Norwegian  mountains,  in  Europe ;  in  New  Zealand, 


300  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

the  southern  Andes,  and  in  various  parts  of  central  Asia  and  western 
America.  There  are  many  thousands  in  the  world,  but  in  the  United 
States  proper  there  are  only  a  few  small  glaciers,  in  the  northern 
Rockies  and  in  the  Sierra  Nevada;  but  as  soon  as  the  Canadian 
border  is  reached  they  commence  to  be  numerous  among  the  moun- 
tains. Glaciers  exist  on  the  line  of  the  Canadian  Pacific  Railway, 
and  from  this  place  to  northern  Alaska  they  are  very  numerous.  No 
part  of  the  world  has  larger  or  more  perfect  valley  glaciers  than 
Alaska.  The  most  noted  of  these  is  the  Muir  Glacier,  north  of 
Sitka;  but  in  this  region,  and  further  north  in  the  Mt.  St.  Elias 
region,  there  are  many  other  grand  valley  glaciers  which  frequently 
terminate  in  the  sea. 

While  glaciers  are  now  scarce  in  this  country,  in  recent 
geological  times  there  have  been  many  in  the  higher  val- 
leys of  the  Rocky  Mountains  and  the  Sierra  Nevada. 
These  existed  at  the  time  of  the  Glacial  Period,  and  have 
left  a  record  of  their  existence  in  the  presence  of  moraines, 
ice-polished  rocks,  and  boulders  which  have  been  taken 
from  the  mountain  tops  down  into  the  valleys.  Where 
now  there  are  only  a  very  few  tiny  glaciers,  scarcely  more 
than  mere  snow  fields,  there  were  formerly  hundreds  of 
well-developed  valley  glaciers.  Because  of  a  change  in 
climate  they  have  gradually  disappeared  from  the  land. 

The  Greenland  Glacier o  —  In  two  parts  of  the  world 
there  are  immense  glaciers  covering  all  the  land  and 
moving  over  both  hill  and  valley.  One  of  these  is  in  the 
Antarctic  region,  surrounding  the  south  pole,  the  other  in 
Greenland,  which  is  almost  entirely  ice  covered.  Almost 
nothing  is  known  about  the  former,  but  many  geologists 
have  visited  the  latter.  In  Greenland  all  but  the  margin 
is  buried  beneath  snow  and  ice,  the  total  area  of  this  great 
glacier  being  about  500,000  square  miles,  an  area  10  times 
as  great  as  the  state  of  New  York.     Both  in  summer  and 


GLACIERS  AND    THE  GLACIAL   PERIOD  301 

winter  snow  falls  upon  the  high  interior  region,  in  which 
there  is  absolutely  no  land,  and  where  the  country  has 
been  buried  to  a  great  depth  and  its  surface  raised  to  an 
elevation  of  10,000  feet  by  the  accumulation  of  snow. 
The  snow  of  the  interior  becomes  compacted  to  ice  at  a 
slight  depth,  and  as  it  accumulates,  slowly  flows  out  in  all 
directions  from  the  interior  toward  the  coast,  north,  south, 
east,  and  west.  It  moves  somewhat  as  a  pile  of  wax  might 
flow  if  a  weight  were  placed  upon  the  top. 


Fia.  160. 

Margin  of  Cornell   Glacier,   Greenland   showing  terminal  moraine.     Black 
layers  of  ice  carry  quantities  of  rock  fragments. 

Near  the  sea  there  are  occasional  mountain  peaks  pro- 
jecting above  the  glacier,  forming  what  the  Greenlanders 
call  a  nunatak ;  and  there  are  also  many  islands  and  penin- 
sulas over  which  the  ice  does  not  extend,  though  upon 
which  there  are  many  small  valley  glaciers.  The  great  ice 
cap  of  Greenland,  riding  over  all  the  land,  and  burying 
even  high  mountain  peaks,  moves  slowly  down  to  the  sea, 


n 


302 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


which  it  enters  at  the  heads  of  bays  or  fjords,  where  it 
breaks  off  in  the  form  of  icebergs.     Between  these  places 

the  ice  rests  on  the 
land  (Fig.  160),  where 
it  melts,  forming 
streams  which  course 
along  between  the 
land  and  glacier.  Now 
and  then  the  water  is 
dammed  into  a  tiny 
lake  into  which  the 
streams  carry  much 
sand  and  clay,  build- 
ing deltas  (Fig.  161). 
Away  from  the 
coast   the   surface  of 


Fig.  161. 


Delta  in  tiny  lake  formed  by  ice  dam  at  mar- 
gin of  Cornell  glacier,  Greenland. 


the  Greenland  ice  cap  is  pure  and  free  from  moraine ;  but 
near  the  sea,  where  peaks  project  above  the  surface,  there 


Fig.  1G2. 
A  scratched  glacial  pebble  from  mnrnine  of  Cornell  glacier. 


GLACIERS  AND   THE  GLACIAL  PERIOD 


303 


are  lines  of  moraine  on  the  glacier,  though  these  are  not 
numerous.  Therefore  nearly  the  entire  surface  of  the 
Greenland  ice  sheet  bears  no  moraine ;  but  in  the  lower 
parts  there  is  considerable  rock  material,  consisting  of 
fragments  varying  in  size  from  grains  of  clay  to  huge 
boulders  (Fig.  160).  Therefore  the  glacier  is  armed  with 
cutting  tools  with  which  it  can  grind  the  land  over  which 
it  slowly  glides.  Indeed 
just  beyond  the  edge  of 
the  ice  are  seen  rock 
surfaces  recently  cov- 
ered, and  now  grooved 
and  polished  by  the  ice- 
scouring  which  they  have 
received.  That  the  ice 
worked  well  is  shown 
by  the  scratches  and 
grooves  of  the  rock, 
which  point  in  the  direc- 
tion from  which  the  gla- 
cier flows.  These  were 
formed  by  the  ice,  which 
pressed  the  boulders 
against  the  bed  rock  and 
dragged  them  along,  as 
we  might  scratch  two  rocks  by  rubbing  them  together. 
The  fragments  brought  in  the  ice  are  often  quite  unlike 
those  on  which  the  edge  of  the  glacier  rests ;  and  hence 
it  is  certain  that  they  have  been  brought  from  some  other 
place. 

Where  the  glacier  enters  the  sea,  the  ground  moraine 
materials  float  off  in  the  icebergs ;  but  where  it  ends  on 


Fig.  163. 
Photograph  of  a  piece  of  floating  ice,  show- 
ing the  relative  amount  of  ice  above  the 
water  surface  (AA)  to  that  below. 


304  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

the  land,  some  goes  off  in  the  streams,  but  much  remains, 
building  terminal-moraine  ridges  and  hills,  as  in  the  case 
of  valley  glaciers.  At  some  recent  geological  time  the 
ice  has  been  more  extensive,  having  formerly  covered  much 
of  the  land  which  now  rises  above  it,  and  perhaps  having 
entirely  obscured  the  Greenland  margin.  The  evidence 
of  this  is  the-  presence  of  moraines  on  the  land,  boulders 
that  have  been  brought  from  some  other  place,  and 
smoothed  and  scratched  rock  surfaces.  In  fact,  these 
are  the  same  as  may  now  be  seen  exactly  at  the  margin 
of  the  glacier. 


Fig.  164. 

Iceberg  ofif  the  North  Greenland  coast. 

Icebergs.  —  When  a  glacier  ends  in  the  sea,  fragments  of  ice  break 
off  and  float  away,  forming  icebergs  ;  and  these  vary  in  size  from  tiny 
pieces  up  to  great  masses,  perhaps  a  mile  in  width,  and  100,  200,  or 
even  300  feet  in  height ;  but  in  the  Arctic,  icebergs  rising  more  than 
100  feet  above  the  water  are  uncommon.  Since  ice  when  floating  has 
8.7  parts  below  the  surface  to  one  above,  a  berg  100  feet  high  sinks 
deep  in  the  water,  and  is  really  an  immense  ice  mass.     In  the  Antarc- 


GLACIEBS  AND   THE  GLACIAL  PERIOD  305 

tic,  some  huge  icebergs  have  been  reported  to  be  so  large  that  they 
have  been  mistaken  for  islands. 

Breaking  off  from  the  glacier  front  with  a  thundering  crash,  sound- 
ing like  a  volley  of  artillery,  they  rock  backward  and  forward,  setting 
the  water  into  commotion  and  causing  powerful  waves  to  move  out 
in  all  directions.  Then  they  float  majestically  away,  being  driven  to 
some  extent  by  the  wind,  but  mainly  by  the  ocean  currents,  changing 
position  now  and  then,  and  once  in  a  while  running  upon  a  shoal, 


,:,^T\^>: 


Fig.  165. 

Map  showing  extension  of  ice  in  eastern  United  States  (by  shading).    The 
heavy  lines  mark  the  position  of  terminal  moraines. 

where  they  remain  until  by  melting  they  can  once  more  float  away. 
Gradually  they  journey  toward  warmer  latitudes,  slowly  melting  until 
they  finally  disappear,  perhaps  thousands  of  miles  from  their  place  of 
birth.  The  ocean  steamers  which  cross  the  Atlantic  to  Europe, 
often  encounter  icebergs  1000  or  2000  miles  from  the  parent  glacier. 
Starting  very  often  with  a  load  of  rock  fragments  in  their  bases, 
they  strew  these  over  the  sea  bed  as  they  slowly  melt. 

Glacial  Period :  Evidence  of  this.  —  Over  northwestern 
Europe  and  northeastern  America,  down  to  the  line  marked 
on  the  accompanying  map  (Fig.  165),  the  surface  presents 


306 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


many  peculiar  appearances.  The  soil  is  not  the  residual 
soil  of  rock  decay  that  commonly  forms  when  rocks  are 
exposed  to  the  weather,  but  generally  consists  of  a  clay 
in  which  there  are  occasional,  or  in  some  cases  numerous 
boulders.  This  is  known  as  boulder  elay^  or  till  (Fig.  166). 
This  till  is  not  found  everywhere,  but  here  and  there, 
especially  in  stream  valleys,  are  beds  of  sand  and  gravel, 
not  unlike  those  now  occurring  where  streams  flow  from 
the  end  of  a  glacier,  as  in  Greenland.  These  deposits 
are  not  strewn  over  the  surface  regularly,  but  in  some 
cases  are  200  or  300  feet  deep,  though  elsewhere  there 
may  be  no  more  than  a  mere  veneer  of  boulder  clay  upon 
the  rock.     Indeed,  in  some  cases  the  surface  is  bare  rock. 

The  pebbles  and  boulders  which  occur  in  the  till  are  not 
the  same  as  those   of  the  rock  in  the  neighborliood,  but 

many  have  been 
brought  from  the 
north,  some  now 
found  in  the 
United  States 
having  come 
from  Canada. 
Moreover  they 
are  scratched, 
quite  like  some 
now  found  in 
the  Greenland 
moraines,  as  if 
they  had  been 
ground  against 
other  rocks ;  and  the  bed  rock  of  the  region  is  scratched 
and  polished  as  in  Greenland  (Fig.  167),  and  the  surface  is 


^ 

1 

1 

y 

J 

'>' 

':-■'  -  ^l^js-^j^^-i 

iiiT^'--^  '^ 

.p^^mm 

■£^^^ 

^«^.^'n^-' 

•^  v  ,-"':*.'•; 

^^^^^  ' 

t 

&5? 

i 

^f^ 

' 

Fig.  166. 
Glacial  till  or  boulder  clay,  Cape  Ann,  Mass. 


GLACIERS  AND   THE  GLACIAL  PERIOD  307 

scoured  into  rounded  outline,  giving  the  form  known  as 
roches  moutonnees,  or  sheep-back  rocks.  The  grooves  and 
scratches  point  toward  the  place  from  which  the  boulders 
have  come,  as  they  do  in  Greenland. 


Fig.  107. 
Rock  surface  in  Iowa,  scratched  by  passage  of  glacier. 

In  the  regions  where  these  peculiarities  occur,  there  are 
also  many  lakes  and  Avaterfalls,  where  streams  have  been 
dammed  or  forced  out  of  their  valleys  by  deposits  of  boulder 
clay  and  gravel.  There  is  an  area  extending  across  our 
country,  in  a  general  westerly  direction,  in  which  these 
conditions  are  found,  but  south  of  which  they  are  absent. 
This  line  of  division  separates  a  land  of  lakes,  falls,  and 
gorges,  or  in  other  words  of  young  streams,  from  one  in 
which  these  are  either  absent  or  very  much  less  abundant. 
Moreover,  on  the  one  side  the  soil  is  boulder  clay,  on  the 
other  residual ;  and  on  the  one  side  the  boulders  are  for- 
eign to  the  region,  and  both  the  pebbles  and  bed  rock  are 
scratched,  while  on  the  other  they  are  not  scratched,  and 
there  are  no  foreign  rocks. 

If  we  were  to  go  to  this  area  of  separation,  we  would  find 
that  it  was  marked  by  a  line  of  hummocky  hills  quite 
closely  resembling  a  terminal  moraine.      This,  which  we 


308 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


may  call  the  terminal  moraine  of  the  Glacial  Period^  ex- 
tends across  country,  passing  up  hill  and  down  ;  and  it 
marks  the  limit  to  which  ice  extended  at  a  recent  period, 
bringing  boulder  clay  and  scratching  and  grinding  the 
rocks  over  which  it  passed.  Because  the  conditions  so 
closely  resemble  those  seen  at  the  margin  of  actual  glaciers. 


Fig.  168. 
Terminal-moraine  hills  near  Ithaca,  N.Y. 

and  because  no  other  agent  will  do  what  has  been  done 
here,  geologists  have  concluded  that  in  a  recent  geological 
period,  a  great  continental  glacier  covered  northeastern  Amer- 
ica and  northwestern  Europe,  as  Greenland  is  now  covered. 

Cause  of  the  Glacial  Period.  —  The /aci  that  the  Glacial  Period  really 
has  existed  cannot  be  doubted;  and  this  being  so,  we  must  believe 
that  there  has  been  a  great  change  in  climate  which  allowed  ice  to 
advance  over  the  country,  and  then  another  which  caused  it  to  with- 
draw.    This  is  in  harmony  with  the  evidence  concerning  the  shrink- 


GLACIERS  AND   THE  GLACIAL  PERIOD 


309 


ing  of  the  Greenland  ice,  and  the  disappearance  of  glaciers  from  the 
mountain  valleys  of  the  west.  Concerning  the  cause  of  this  there  has 
been  much  discussion ;  but  like  many  scientific  facts,  the  true  expla- 
nation has  not  yet  been  found  and  proved.  We  have  many  theories, 
but  it  is  difficult  to  prove  one  and  disprove  the  others,  for  the  question 
deals  with  conditions  that  are  past,  and  the  ejects  of  which  only  can 
be  studied. 


Fig.  169. 
A  boulder-strewn  moraine  hill,  Cape  Ann,  Mass. 


Glacial  Deposits.  —  When  the  ice  front  stood  at  any  one 
place  for  a  considerable  length  of  time,  it  dragged  ground 
moraine  down  to  its  edge  and  piled  it  up  along  the  margin 
as  a  terminal  moraine.  There  are  many  such  moraines  in 
the  United  States  and  Canada;  for  not  only  did  the  ice 
advance  to  and  stand  at  the  line  described  above,  but  as 
it  slowly  melted  from  the  land,  its  front  stood  along  lines 
further  and  further  north,  and  each  time  that  it  halted  a 
moraine  was  built.  These  terminal  moraines  are  among 
the  most  characteristic  land  forms  in  this  country  (Figs. 


310  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

168  and  169).  They  consist  of  a  dump  of  rock  fragments, 
varying  in  size  from  boulders  to  clay,  and  their  surface 
outline  is  irregular  and  hummocky,  as  any  mass  of  earth 
would  be  if  dumped  without  order.  There  are  small  hills, 
saucer-shaped  valleys,  ridges,  and  in  general  an  exceed- 
ingly irregular  surface.  They  are  chiefly  composed  of  till 
brought  by  the  glacier  and  laid  down  without  assortment, 
and  hence  without  stratification;  but  they  also  contain 
beds  of  stratified  sand  and  gravel,  which  were  deposited 
by  water  furnished  by  the  melting  ice.  Therefore  these 
moraines  are  complex  in  structure  as  well  as  outline. 

When  the  ice  advanced,  it  carried  with  it  a  load  of  rock 
fragments  which  it  held  as  a  ground  moraine ;  and  when 
finally  it  disappeared,  this  was  left  as  a  sheet  of  boulder 
clay  strewn  over  the  surface,  sometimes  as  a  thin  layer 
and  sometimes  in  very  thick  beds.  When  the  ice  crossed 
valleys,  till  was  often  dragged  down  into  these  and  left 
there,  so  that  some  valleys  have  been  entirely  buried, 
while  others  have  a  filling  of  perhaps  100  or  200  feet  of 
boulder  clay.  Some  of  the  till  is  very  bouldery  (Fig.  169) ; 
but  in  other  cases,  where  the  supply  of  hard  rock  was 
scanty,  there  are  few  large  rocks  (Fig.  168),  and  in  some 
of  the  till  no  boulders  are  found.  This  till  has  various 
forms,  but  generally  it  is  a  sheet  extending  uniformly  over 
the  surface  of  the  rock  on  which  it  rests.  Tlie  streams 
from  the  ice  have  also  deposited  sheets  of  gravel  and  built 
low  hills  of  various  kinds,  and  the  land  which  has  been 
ice  covered  is  therefore  strewn  with  various  kinds  of 
glacial  deposits. 

Effects  of  the  Glacier.  —  While  it  existed  the  ice  did 
much  work.  It  has  moved  materials  from  one  place  to 
another ;  it  has  swept  off  the  old  soil  and  replaced  it  by 


GLACIERS  AND   TUE  GLACIAL  PERIOD  311 

a  new  kind ;  it  has  worn  and  lowered  many  of  the  hill- 
tops, deepened  some  of  the  valleys,  and  partly  or  entirely 
filled  others ;  and  it  has  left  the  land  surface  quite  differ- 
ent from  the  way  in  which  it  was  found.  Still  with  all  its 
work,  it  has  not  been  able  to  erase  the  larger  preglacial 
hills  and  valleys,  but  merely  to  modify  them. 

The  most  important  effect  of  glacial  action  has  been 
upon  the  drainage.  When  the  ice  stood  as  a  barrier  on 
the  land  it  stretched  across  many  stream  valleys  whose 
slope  was  toward  the  north,  and  these  were  then  trans- 
formed to  lakes,  which  being  prevented  from  flowing  as  the 
land  sloped,  were  forced  to  seek  some  other  outlet.  There- 
fore many  north-flowing  streams  for  awhile  flowed  south- 
ward, and  some  of  these,  cutting  down  the  bariiers  at 
their  outlet,  lowered  them  so  greatly  that  when  the  ice 
disappeared,  the  slope  of  the  land  no  longer  led  them 
northward,  and  they  continued  to  flow  in  their  reversed 
direction. 

One  of  the  best  cases  of  this  is  found  in  the  headwaters  of  the 
Alleghany,  which  before  the  Glacial  Period  belonged  to  the  St.  Law- 
rence drainage,  but  now  joins  the  Ohio.  While  these  lakes  existed, 
beaches  and  deltas  were  built  in  them ;  and  since  the  water  has  now 
disappeared  from  the  basins,  we  may  often  see  these  old  lake  deposits 
clinging  to  the  hillsides.  Not  only  did  these  conditions  exist  near 
the  southern  terminal  moraine,  but  along  the  margin  of  the  ice,  where- 
ever  it  stood  on  the  land.  Hence  as  the  glacier  withdrew  northward, 
as  it  was  melting  from  the  country,  the  zone  of  temporary  glacial  lakes 
extended  further  northward. 

Before  the  ice  came,  the  land  was  carved  into  hills  and 
valleys,  and  streams  occupied  the  surface ;  but  while  the 
glacier  existed  they  were  for  a  time  extinguished.  As 
soon  as  the  ice  left,  the  streams  again  occupied  the  land, 
but  only  to  find  the  old  valleys  considerably  altered.     In 


312  FIRST  BOOK  IN  PHYSICAL   GEOGRAPHY 

some  cases,  as  in  Ohio  and  the  other  level  states  of  the 
plains,  the  valleys  were  entirely  obliterated  by  glacial 
deposits,  and  new  ones  had  to  be  started  by  young  streams 
flowing  on  the  plains  of  glacial  deposit.  In  other  cases, 
and  particularly  in  the  moderately  hill}^  countries,  streams 
were  sometimes  caused  to  leave  their  old  valleys  and  carve 
new  ones.  Or  they  were  turned  from  the  old  channels 
by  glacial  deposits  and  caused  to  cut  gorges  in  the  rock, 
or  in  the  till,  to  one  side  of  their  former  course.  There- 
fore, they  were  locally  rejuvenated ;  and  hence  it  is  that 
so  many  streams  in  the  glaciated  country  flow  in  gorges 
for  a  part  of  the  distance,  and  that  in  these  there  are 
numerous  waterfalls  (Chapter  XVI). 

With  equal  frequency  dams  of  moraine  have  been  thrown 
across  the  stream  course,  forming  lakes.  Upon  the  irregu- 
lar sheet  of  till,  the  terminal  moraine  and  the  gravel  hills 
of  United  States  and  Canada,  there  are  probably  more  than 
100,000  lakes  and  ponds.  Practically  all  the  lakes  of 
northern  United  States  and  Canada  are  the  result  of 
glacial  deposits,  either  because  their  surface  was  dotted 
with  saucer-shaped  depressions,  or  else  because  they  have 
formed  dams  across  stream  valleys.  This  is  one  of  the 
reasons  for  the  existence  of  the  Great  Lakes,  though  they 
were  undoubtedly  caused  in  part  by  the  action  of  ice  in 
digging  valleys  deeper,  and  in  part  by  changes  in  the  level 
of  the  land  which  have  formed  rock  dams.  Even  with 
these  causes,  much  of  the  area  of  the  Great  Lakes  is 
due  to  the  effect  of  glacial  deposits  which  have  partly 
choked  old  preglacial  valleys. 


CHAPTER   XVIII 

SEA   AND   LAKE    SHORES 

Difference  between  Lake  and  Sea  Shores.  —  While  there 
are  many  differences  in  detail,  the  shores  of  sea  and  lake 
so  closely  resemble  one  another  that  they  may  be  con- 
sidered together.  In  both  we  find  cliffs  and  beaches, 
promontories  and  bays ;  and  in  each  there  are  many 
differences  between  these  from  point  to  point.  In  both 
places  there  are  waves  constantly  at  work,  and  these  differ 
in  force  from  place  to  place ;  and  there  are  also  wind- 
formed  currents  in  each.  But  in  several  ways  they  differ : 
no  great  circulation,  like  that  of  the  ocean  currents,  is 
found  in  lakes,  nor  are  there  well-developed  tides.  Be- 
sides these,  animal  life  is  less  abundant  in  lakes  than  in 
the  sea,  and  therefore  certain  kinds  of  shores  which  are 
constructed  by  ocean  animals  are  never  found  in  lakes. 
On  the  other  hand,  the  action  of  plant  life  is-  different  in 
the  two  bodies,  being  more  important  in  the  fresh  water. 

Form  of  the  Coast.  —  If  we  pass  along  an  extensive 
stretch  of  coast  line,  we  find  many  differences  as  we  pro- 
ceed. For  instance,  the  coast  of  New  England,  north  of 
Cape  Cod,  is  chiefly  rocky,  and  exceedingly  irregular. 
There  are  tens  of  thousands  of  islands  and  peninsulas, 
great  and  small,  and  as  many  bays,  harbors,  and  estuaries. 
To  go  on  foot  along  the  margin  of   such  a  shore  for   a 

313 


314         FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

distance  of  a  dozen  miles,  as  measured  from  headland  to 
headland,  may  require  one  to  walk  not  less  than  100 
miles,  in  passing  around  the  bays  (Fig.  176  and  Plate  19). 
While  rocky  in  general,  it  would  be  found  by  such  a  jour- 
ney that  the  abrupt  sea  cliffs  and  headlands  often  enclose 
beaches,  either  of  pebbles  or  sand,  and  that  salt  marshes 
and  mud  flats  line  the  head  of  many  of  the  bays. 

Passing  south  of  Cape  Cod,  it  is  found  that  the  coast 
becomes  less  abrupt  and  rocky,  and  at  the  same  time  less 
irregular,  until  on  the  Carolina  coast  the  shore  is  a  series 
of  sand  bars  and  beaches,  with  no  rocks  and  few  irregu- 
larities, excepting  those  formed  by  the  sand  bars.  Still 
further  south,  on  the  southern  end  of  Florida,  the  coast 
again  becomes  irregular ;  but  here  the  islands,  or  as  they 
are  called,  the  ket/s,  are  made  of  coral  fragments.  Carry- 
ing the  examination  to  more  distant  lands,  we  see  that 
while  the  coast  of  Europe  and  the  northern  coasts  both  of 
western  and  eastern  America  are  exceedingly  irregular, 
the  west  coast  of  South  America,  although  rocky,  is  so 
uniformly  straiglit  that  good  harbors  are  scarce.  These 
differences  are  due  to  perfectly  natural  causes,  most  of 
which  can  be  simply  explained. 

Sea  Cliffs.  — Beating  against  the  coast,  the  waves,  armed 
with  pebbles  and  sand,  are  cutting  into  the  land.  Tliey 
gradually  wear  away  the  hardest  rocks,  and  on  many 
coasts  have  made  important  changes,  even  since  man  has 
inhabited  tliem.  Where  waves  are  at  work  violently, 
whether  in  lake  or  sea,  their  resistless  sawino*  at  the  rocks 
cuts  them  into  the  form  of  cliffs  (Fig.  170),  which  if  the 
rock  is  hard,  may  rise  nearly  vertically,  or  if  soft,  with 
less  steep  slopes;  for  then  the  sand,  clay,  or  gravel  will 
slide  down  until  it  can  come  to  rest.     With  the  action 


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SEA   AND  LAKE  SHORES 


315 


of  the  waves  and  their  allies,  the  wave  and  wind  currents, 
not  only  is  the  rock  cut  away,  but  it  is  removed,  leaving 
the  cliff  open  to  fresh  attacks.  Just  so  long  as  this  is 
done  the  cliff  will  maintain  its  steepness;  for  the  waves 
saw  along  a  narrow  zone  near  sea  level,  and  thus,  under- 
cutting the  cliff  (Fig.  171),  perhaps  forming  sea  caves, 
undermine  the  rock  above,  causing  it  to  fall  because  its 


Fig.  170. 
A  sea  cliff  on  south  shore  of  Bermuda. 


support  is  removed.  So,  gradually  by  this  undermining 
action,  fragments  of  the  cliff  face  are  caused  to  fall,  and 
slowly  it  moves  backward,  the  falling  fragments  being 
carried  away  by  the  waves.  If  the  time  comes  when  the 
materials  cannot  be  taken  away,  the  waves  cease  to  cut 
into  the  cliff,  and  under  the  influence  of  weatherinof  it 
gradually  loses  its  steepness. 

Sea  cliffs  of  great  size,  rising  hundreds  of   feet  out  of 
the  water,  are  found  in  the  open   ocean  where   the   full 


316 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHT 


force  of  powerful  waves  can  be  exerted  against  the  shore ; 
but  similar  though  smaller  cliffs  occur  in  lakes  (Fig.  172), 
and  also  in  the  enclosed  bays  of  the  seashore,  where  smaller 
waves  are  generated.  Their  form  varies  greatly  with  the 
kind  of  rock  out  of  which  the  coast  is  made ;  some  shores 
crumble  so  rapidly  under  weathering  that  they  are  never 
vertical,  and  this  is  particularly  true  where  the  material  is 
soft  sand  or  gravel.  In  other  cases  the  rocks  are  so  hard 
that  they  can  be  cut  into  the  form  of  a  cliff,  which  main- 


FiG.  171. 
Wave-cut  islands,  Bermuda,  showing  undercutting  action  of  waves. 


tains  the  form  of  a  precipice  for  a  long  time.  Sometimes 
these  vertical  cliffs  are  several  hundred  feet  in  height,  ris- 
ing directly  from  the  water ;  but  in  other  cases  the  shore  con- 
sists of  rounded  and  somewhat  irregular  outlines.  The  sea 
cliff  is  the  natui'al  result  of  wave  work  where  they  are  free 
to  cut  as  they  will.  One  may  therefore  expect  that  this 
would  be  not  only  the  grandest  but  also  the  commonest 
of  seasliorc  features ;  but  the  latter  is  certainly  not  true. 


SEA  AND  LAKE  SHORES  317 

The  Beach.  —  When  the  waves  are  cutting  against  a 
cliff  and  wearing  it  back,  they  are  obtaining  materials 
which  must  be  disposed  of  if  they  would  continue  their 
work  of  cutting.  As  has  been  explained,  the  materials 
wrested  by  the  waves  are  removed  by  the  undertow,  the 
wind-formed  currents,  and  the  swash  of  the  surf,  which 
rushes  upon  the  land  at  a  slight  angle  to  the  direction 
in  which  the  coast  extends.  By  the  first  of  these  some 
of  the  material  is  carried  off  shore,  and  by  the  others  along 
shore;  but  to  that  burden  which  the  waves  themselves 
take,  is  added  one  imposed  upon  them.  Weathering,  wind 
action,  and  rivers,  are  furnishing  other  materials  to  the 
waves,  and  these  supplies  sometimes  become  so  great  that 
the  waves  are  overburdened,  and  cannot  perform  the  great 
task  of  removal  thrust  upon  them. 

For  instance,  along  the  entire  coast  of  the  United  States 
south  of  New  York,  excepting  at  the  southern  end  of  Flor- 
ida, the  waves  have  more  material  than  they  can  carry  off. 
Hence  it  is  that  the  great  ocean  waves  that  beat  against 
the  Carolina  coast  are  not  cutting  cliffs  in  the  soft  rock, 
but  are  breaking  upon  sand  beaches,  often  at  distances 
of  several  miles  from  the  real  shore,  fiom  which  the  outer 
beach  bars  are  separated  by  lagoons  and  marshes.  Tliese 
bars,  such  as  those  forming  Cape  Hatteras,  have  been  built 
up  by  the  waves  out  of  materials  which  were  furnished 
them,  but  which  they  could  not  carry  away.  Being  forced 
to  lay  down  their  burdens,  they  have  built  hundreds  of 
miles  of  bars ;  and  then  the  winds,  taking  the  sand  from 
the  beaches,  have  piled  it  up  in  the  form  of  sand  dunes  or 
sand  hills.  Therefore  the  bars  are  partly  wave-formed 
and  partly  wind-formed ;  but  the  supply  has  been  furnished 
chiefly  by  the  rivers. 


318 


FIRST  BOOK  OF  PnVSICAL    GEOGRAPHY 


Along  an  irregular  rocky  coast,  like  that  of  Maine,  the 
waves  have  an  easier  task.  They  have  harder  rocks  against 
which  to  work,  and  hence  get  less  of  a  load  to  carry,  and 
the  rivers  entering  the  sea  there  do  not  transport  so  much 
sediment.  The  coast  is  more  irregular,  and  because  of 
this  the  waves  beat  against  the  relatively  few  headlands, 
wear  fragments  off,  carry  some  out  to  sea,  and  drive  others 

along  shore  un- 
til an  indenta- 
tion of  the  coast 
is  reached, 
where  the  load 
is  dropped,  be- 
cause upon  en- 
tering a  bay  or 
harbor,  the 
power  of  llic 
waves  decreases. 
Hence  upon  this 
coast  there  are 
many  little 
pocket  beaches 
of  sand,  pebbles 
or  boulders  that  have  been  wrested  from  the  neighbor- 
ing cliffs  and  driven  into  the  depressions.  In  the  larger 
indentations  there  are  extensive  beaches  of  sand  and  peb- 
bles (Fig.  110).  Further  up,  near  the  head  of  bays,  where 
the  waves  are  always  tiny,  mud  flats  exist,  because  here 
even  clay  cannot  be  carried  away,  and  that  which  the 
waves  wear  off,  as  well  as  that  furnished  by  weathering 
and  streams,  is  accumulated  there. 

By  this  means  the  bays  and  other  indentations  of  the 


Fig.  172. 
Wave-cut  cliff  with  beach  in  small  bay,  Lake  Superior. 


SEA  AND  LAKE  SHORES 


319 


coast  are  gradually  filling  up.  They  become  a  dumping 
ground  for  the  wave-derived  materials,  as  well  as  those 
coming  from  the  land  itself.  Because  of  this  fact  some 
harbors  have  been  rendered  useless,  while  upon  others  it 
is  necessary  to  spend  large  sums  of  money  every  year,  in 
order  to  keep  them  deep  enough  for  large  ships  to  enter. 
Therefore  the  double  action  of  cutting  the  cliffs  and  fill- 
ing the  bays  results  in  production  of  more  regular  coasts. 


Fig.  173. 
Bar  built  across  bay,  Cape  Breton  Island,  Nova  Scotia. 

One  of  the  first  steps  in  harbor  filling  is  the  forming  of  a 
bar  across  the  mouth  of  the  indentation.  (Figs.  173  and 
174).  Driven  along  shore,  the  materials  are  dropped, 
causing  the  water  near  the  mouth  of  the  bay  or  harbor 
to  become  shallower.  Later  as  more  is  added,  the  bar 
reaches  the  surface,  and  finally  stretches  from  side  to  side, 
perhaps  completely  enclosing  it,  and  transforming  it  to 
a  pond,  though  more  commonly  a  small  opening  is  main- 
tained tlirougli  which  the    tide   ebbs  and  flows.      Along 


320 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


the  United  States  coast  there  are  thousands  of  such  bar 
beaches,  some  of  them  miles  in  length. 

Wave-carved  Shores.  —  Besides  constructing  these  wave- 
built  forms,  and  cutting  the  sea  cliffs,  the  waves  have  done 
much  work  of  carving  rocky  shores  into  irregular  outline. 
Wherever  there  is  a  softer  layer  between  harder  walls,  the 
waves  will  find  this  and  eat  into  it  more  rapidly  (Fig.  175). 
Hence  a  rocky  coast  on  which  the  materials  vary  in  kind, 


Fig.  174. 
Map  of  a  part  of  Martha's  Vineyard,  showing  arms  of  the  sea  cut  off  by  bars. 

is  liable  to  be  very  irregular,  consisting  of  alternating 
headlands  and  minor  indentations.  But  these  do  not 
become  very  great,  because  as  they  increase  in  depth,  the 
waves  that  enter  liave  less  power,  and  they  become  then 
a  place  of  deposit,  and  have  beaches  built  at  their  heads, 
which  protect  the  rocks  from  the  further  attack  of  the 
waves. 

It  lias  sometimes  been  stated  that  such  great  bays  as  the 
Chesapeake,  and  the  straits,  bays,  and  harbors  of  the  coast 
of  New  England,  have  been  cut  out  by  waves  and  tides; 


SEA  AND  LAKE  SHORES 


321 


but  this  is  certainly  not  true,  for  waves  cease  to  be  able 
to  cut  when  they  enter  bays,  and  tides  have  not  the  power 
to  do  the  cutting.  Indeed  careful  study  shows  that  such 
bays  are  being  filled,  not  deepened  and  enlarged  by  tides 
and  waves ;  and  so  it  is  necessary  to  look  for  another 
cause  to  explain  these  greater  irregularities. 

A  Sinking  Coast.  —  The  elevation  of  the  land  is  sub- 
ject to  frequent  change, 
and  some  places  are  ris- 
ing, others  sinking,  while 
some  have  recently 
changed  in  one  or  the 
other  of  these  directions. 
If  the  land  should  sink 
near  the  coast,  one  of  the 
effects  would  be  to  cause 
the  sea  to  enter  the  val- 
leys, transforming  tliem  to 
bays,  if  they  were  broad, 
or  to  fjords  if  narrow. 
Should  the  submergence 
continue,  in  time  the  water 
would  rise  over  low  di- 
vides and  transform  them 
to  straits,  and  make  some 
hills  into  islands,  others  into  promontories  and  capes.  The 
coast  would  then  become  very  irregular  (Figs.  176  and 
180,  and  Plate  19). 

This  description  of  what  would  happen  may  be  applied 
to  the  eastern  coast  of  North  America  and  the  western 
shores  of  northern  Europe.  Here  there  are  islands  which 
resemble  hilltops,  straits  between  capes  and  islands,  and 


Fig.  175. 

A  wave-carved  indentation  on  shore  of 
Cape  Ann,  Mass.  Small  bay  formed 
where  a  soft  trap  rock  crosses  the 
harder  granite. 


322 


FIBST  BOOK  OF  PHYSICAL   GEOGRAPHY 


bays,  harbors,  and  fjords  which  end  in  river  valleys  on  the 
land.  In  fact  the  resemblance  is  so  close  to  what  actually 
would  happen  if  the  land  should  be  partly  drowned,  that 
it  is  one  of  the  strong  proofs  that  such  sinking  actually 
has   occurred.      For 


instance,  the  strait 
separating  Great 
Britain  from  Europe 
appears  to  represent 
a  low  divide  sunk 
beneath  sea  level. 
The  Hudson  River, 
into  which  the  tide 
rises  above  Albany, 
appears  to  be  a  river 
valley  carved  on  the 
land.  The  Bay  of 
St.  Lawrence  and 
its  tributaries,  and 
the  Chesapeake  and 
its  brandies,  appear 
to  be  nothing  more 
than  land  valleys 
now  partly  beneath 
the  sea  (Fig.  146). 

A  sinking  coast 
brings  the  sea  water 
in  contact  with  the  hard  rock  of  the  land,  and  therefore 
reduces  the  amount  of  material  which  waves  can  cut.  It 
makes  the  shore  line  more  irregular,  and  therefore  fur- 
nishes depressions  into  which  the  waves  can  drop  their  load. 
It  causes  the  water  to  rise  higher  and  higher,  submerging 


Cochenoe  Id, 


Fig.  17G. 

Map  of  part  of  Connecticut,  showing  present 
outline  of  coast  witii  bays,  peninsulas,  and 
islands,  and  (by  shading)  the  similar  outline 
which  would  result  if  the  land  should  sink  a 
hundred  feet  more. 


SEA  AND  LAKE  SHORES  323 

the  beaches  which  the  waves  commence  to  build.  There- 
fore, for  these  various  reasons,  a  sinking  coast  is  liable  to 
be  not  only  an  irregular  one,  but  also  one  of  numerous 
cliffs  and  few  beaches.  Thus  on  the  coast  of  Greenland, 
which  is  now  sinking,  there  are  very  few  beaches;  but 
high  cliffs  rise  directly  out  of  quite  deep  water.  In  lakes 
the  rising  of  the  water  for  any  reason  produces  the  same 
results,  as  may  be  seen  on  the  south  shore  of  the  Great 
Lakes. 

A  Rising  Coast.  —  A  rising  coast  is  less  irregular  because 
the  sea  bottom  is  much  smoother  than  the  land.  It  is 
covered  with  soft  mud  and  sand,  and  hence  the  waves 
obtain  a  great  load,  and  generally  more  than  the}^  can 
carry  off,  so  that  bars  are  often  built,  as  in  the  case  of 
the  coast  of  Texas,  which  has  been  recently  elevated.  The 
western  coast  of  South  America  is  also  rising,  and  this  is 
the  reason  why  there  are  so  few  harbors  and  such  a  won- 
derfully straight  coast.  In  lakes  whose  level  is  being 
lowered,  similar  though  much  less  pronounced  effcc  Is 
are  produced. 

Marshes. — On  many  shores,  such  as  those  of  eastern 
United  States,  there  are  extensive  marshy  plains  i;i  the 
protected  bays  and  estuaries.  These  are  seen  on  the  New 
England  coast  as  well  as  behind  the  sand  bars  of  the  moie 
southern  states.  Salt  marshes  are  formed  by  a  partial, 
and  in  some  cases  a  complete,  filling  up  of  these  enclosed 
areas,  which  are  out  of  reach  of  the  ocean  waves.  Here 
rain  and  rivers  wash  materials  into  the  arms  of  the  sea, 
and  to  this  deposit  is  added  sediment  driven  in  by  the 
waves  and  borne  by  the  tidal  currents.  Settling,  the 
accumulation  slowly  raises  the  sea  floor  until  the  depth  is 
shallow  enough  for  certain  plants  to  take  root,  and  then 


324  FIBST  BOOK  OF  PHYSICAL   GEOGRAPHY 

these  raise  it  still  higher,  partly  by  entangling  sediment 
and  causing  it  to  settle,  and  partly  by  their  death. 

The  salt  marsh  grass  can  grow  only  in  places  where  it  is  at  some 
time  exposed  to  the  air,  though  it  must  also  at  some  time  receive  a 
bath  of  salt  water.  Gradually  the  surface  rises  to  the  level  of  the 
high  tide,  becoming  then  a  remarkably  level  plain,  through  which  ex- 
tend numerous  channels  occupied  by  the  rising  tide,  which  fills  them 
and  then  overflows  the  plain  with  a  sheet  of  salt  water.  In  time  the 
marshes  rise  even  above  this  level  and  then  become  dry  land.  Before 
this  stage,  in  many  countries,  men  have  built  dykes  to  keep  out  the 
salt  water,  and  have  established  farms  and  even  towns  upon  a  plain 
which  is  really  below  the  level  of  the  high  tide.  Both  in  England 
and  America  there  has  been  much  land  reclaimed  in  this  way,  and 
large  areas  of  Holland  were  once  salt  marsh. 

The  mangrove  (Fig.  77),  a  tree  which  can  grow  with  its  roots  in 
salt  water,  is  building  similar  marshes,  though  in  this  case  tree-cov- 
ered. In  this  country  these  are  found  in  the  bays  of  Florida,  and 
they  exist  on  other  subtropical  as  well  as  tropical  coasts.  In  lakes, 
similar  treeless  and  tree-covered  marshes  are  built  by  the  aid  of  plant 
growth ;  and  many  small  ponds  are  bordered  by  such  swamps,  while 
some  have  been  entirely  replaced  by  them.  Both  in  sea  and  lake 
these  marsh  lands  can  develop  only  where  the  waves  are  not  active 
enough  to  remove  the  clay  or  sand  in  which  the  vegetation  takes  root. 

Coral  Reefs.  —  Where  conditions  are  favorable,  animal 
life  in  the  sea  exists  in  great  luxuriance ;  and  particularly 
is  this  true  in  the  shallow  water  in  and  near  the  tropical 
regions,  where  the  reef-building  corals  abound.  These 
animals  can  thrive  only  where  the  water  is  warm  and  the 
temperature  never  lower  than  68°  or  70°,  the  depth  not 
more  than  150  feet,  and  where  they  are  not  exposed  to 
the  air  between  tides.  In  addition  to  this,  the  animals 
must  have  a  good  supply  of  food,  and  hence  they  are  gen- 
erally found  where  ocean  currents  exist.  If  the  water  is 
muddy,   or  if  fresh   water   enters   the   sea,  they   cannot 


SEA  AND  LAKE  SHORES 


325 


abound.  Hence  reef-building  corals  need  a  combination 
of  unusually  favorable  conditions,  and  their  distribution 
in  abundant  colonies  is  not  great.  Where  these  favorable 
conditions  are  combined,  the  abundance  of  coral  and  other 
lime-secreting  animals  is  marvellous  (Fig.  79). 

Corals  may  abound  in  the  shallow  waters  near  the  land, 
along  which  they  build  fringing  reefs  rising  nearly  to  sea 
level.  Or  they  may  build  a  reef  just  off  shore,  which  is 
then  known  as  a  barrier  reef.     They  grow  so  luxuriantly 


Fig.  177. 
Beach  on  coast  of  Florida.    Position  of  coral  reef  shown  by  line  of  breakers. 


that  they  build  the  bottom  up  so  near  the  surface  that 
the  waves,  coming  upon  the  shore,  break  over  the  reef, 
forming  a  great  line  of  surf,  in  which  the  animals  thrive 
because  the  water  is  kept  in  commotion,  thus  causing  a 
constant  passage  of  food,  which  must  come  to  the  corals, 
since  they  are  firmly  anchored  in  place.  Upon  passing 
over  the  reefs,  the  waves  break  off  fragments  of  coral,  or 
tear  away  entire  masses,  and  drive  them  ashore  upon  the 
beach,  where  they  are  ground  up  (Fig.  177). 

This  coral  sand,  taken  by  the  wind,  may  be  blown  into 


326  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

the  form  of  hills;  and  hence  land  will  be  actually  con- 
structed out  of  coral  fragments.  The  southern  end  of 
Florida  and  the  Bahama  Islands  are  made  of  coral  sub- 
stance ;  and  the  entire  area  of  the  Bermudas,  above  sea 
level,  is  made  of  shell  sand  derived  from  reefs  and  built 
into  the  form  of  hills  by  the  action  of  the  wind.  Along 
the  shores  of  northern  Australia  there  is  a  reef,  called  the 
Great  Barrier  Reef,  which  is  over  1000  miles  in  length ; 
and  from  this  is  being  supplied  a  vast  amount  of  material 
out  of  which  rock  is  being  built. 

In  the  southern  Pacific  there  are  many  coral  islands  far  away  from 
any  land,  and  having  the  form  of  a  more  or  less  perfect  ring.  These 
are  known  as  atolls,  and  the  coral  ring  rises  a  number  of  feet  above  the 
water  surface,  partly  because  the  waves  have  thrown  fragments  above 
sea  level,  but  chiefly  because  the  wind  has  blown  the  coral  sand  from 
the  beach  into  the  form  of  low  hills.^ 

Islands.  —  Islands  are  either  new  land  built  up  in  llie 
sea  or  else  remnants  of  old  land  partly  destroyed.  The 
former  may  be  called  islands  of  construction^  the  latter 
islands  of  destruction.  By  far  the  greater  number  of 
islands  exist  near  the  sea  coast,  but  not  a  few  are  found 
in  mid-ocean.  In  size  they  vary  from  tiny  bits  of  land, 
covered  at  high  tide,  to  immense  islands,  like  that  of 
Greenland. 

By  Construction. — Islands  maybe  iwz7^  by  various  means. 
Near  deltas  they  are  often  formed  because  the  waves  cannot 
remove  all  the  sediment  out  to  sea.  Along  such  coasts  as 
that  of  eastern  United  States,  particularly  along  the  shore 
line  of  the  southern  states,  where  the  waves  have  more 

1  For  the  theories  to  account  for  these  interesting  atolls  a  book  on 
geology  may  be  consulted. 


SEA  AND  LAKE  SHOIiES 


327 


sediment  than  they  can  dispose  of,  the  combined  action  of 
winds  and  ivaves  builds  a  great  many  sand  bars,  which  are 
true  islands,  generally  very  long  and  narrow.  Along  our 
coasts  there  are  many  thousands  of  these,  mostly  small,  but 
sometimes  a  score  or  two  of  miles  in  length.  Such  bars 
can  be  made  only 
near  laud,  where 
the  water  is  shal- 
low. Animals  are 
also  building 
islands ;  the  coral 
reefs  and  the 
atolls  just  de- 
scribed are  of  this 
class.  Along  the 
coast  of  Bermuda 
there  are  many 
tiny  islands  that 
have  been  built 
by  serpula  (a  worm  that  makes  a  calcareous  shell)  into 
the  true  ring  form  of  the  atoll  (Fig.  178). 

Islands  may  be  constructed  b}^  the  elevation  of  an  irregu- 
lar sea  bottom ;  for  then  higher  parts  of  the  bed  may  be 
raised  above  the  sea,  forming  islands.  Or  the  folding  of 
the  sea  bed  through  the  formation  of  mountains  may 
also  raise  parts  above  the  sea  level.  This  is  the  origin  of 
many  of  the  larger  islands  of  the  world.  The  East  and 
West  Indies  are  parts  of  mountains  not  now  raised  high 
enough  to  form  a  portion  of  the  continents  near  which 
they  lie. 

The  Hawaiian  Islands  are  an  instance  of  this ;  for 
between  the  several  islands  of  this  group  there  extends  a 


Fig.  178. 

Serpula  atolls,  Bermuda. 


328  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

ridge  of  upfolded  sea  bottom,  evidently  a  true  mountain 
range  beneath  the  sea.  The  peaks  which  rise  from  the 
crest  of  this  ridge  are  illustrations  of  another  type  of  con- 
structed islands,  the  volcanic.  By  outpouring  of  molten 
rock  these  peaks  have  been  raised  as  islands,  and  in  this 
case  also  there  are  many  similar  elevations  not  yet  raised 
above  the  surface.  All  the  isolated  islands  of  the  open 
ocean,  excepting  those  made  of  coral,  are  volcanic  peaks ; 
and  it  is  probable   that  most,   if   not   all  of  the  isolated 


Fig.  179. 
Islands  on  shore  of  Bermuda,  cut  from  the  land  (on  right)  by  wave  action. 

coral  atolls  of  the  mid-ocean  have  been  built  by  animals 
upon  platforms  constructed  by  volcanic  causes. 

Bi/  Destruction.  —  By  far  the  greatest  number  of  islands 
are  caused  by  the  destruction  of  land.  For  instance,  the 
waves  beating  against  the  shore  and  wearing  it  back,  may 
leave  some  parts  standing  for  awhile,  thus  forming  islands 
(Fig.  179).  Such  islands  can  never  be  large,  being  really 
tiny  fragments  of  the  coast  line  not  yet  destroyed. 

The  sinldyig  of  the  land  accounts  for  the  greatest  number 
of  instances  of  islands  of  destructive  origin.  The  partial 
drowning  of  an  irregular  coast  transforms  the  shore  into 
an  exceedingly  irregular  area  of  promontories  and  islands, 


SEA  AND  LAKE  SUORES  320 

the  latter  being  produced  when  the  hilltops  are  sur- 
rounded by  water.  The  subsidence  of  the  Bermudas  has 
caused  the  large  number  of  islands  seen  there  (Fig.  180). 
Those  extending  along  the  coast  of  Maine  are  due  to  this 
cause  also  (Plate  19),  and  the  vast  number  along  the  shores 
of  northeastern  America,  some  of  which  are  of  large  size, 
represent  merely  high  peaks  of  the  old  land  now  partly 
drowned  in  the  sea. 


Fig.  180. 
Islands  in  the  Bermudas,  due  to  sinking  of  the  land. 

Some  islands  are  being  increased  in  size  by  the  causes 
which  constructed  them ;  but  as  soon  as  these  causes  cease, 
they,  like  other  islands,  are  attacked  by  the  waves.  Since 
they  are  surrounded  on  all  sides  by  water  they  can  be 
attacked  in  all  directions,  and  this  attack  will  be  active 
on  all  sides,  especially  if  they  stand  well  out  in  the  open 
ocean.     Gradually  then  they  disappear,  and  hence  if  new 


330         FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

supplies  be  not  added,  any  island  will  in  time  be  destroyed. 
The  volcanic  peaks  rising  in  the  sea  grow  in  size  as  long 
as  the  volcano  erupts ;  coral  keys  grow  as  long  as  the 
animals  thrive ;  and  a  sand  bar,  built  by  the  waves,  in- 
creases just  as  long  as  the  waves  bring  more  than  they 
can  carry  away ;  but  when  these  conditions  cease,  as  they 
may  in  time,  the  island  is  doomed  to  destruction. 


Fig.  181. 
Island  joined  to  mainland  by  two  bars,  Cape  Breton,  Nova  Scotia. 

Promontories.  —  What  has  been  said  about  islands  applies  to  prom- 
ontories and  to  capes,  which  are  merely  small  promontories.  Some 
are  built  by  the  waves,  others  may  be  caused  by  the  joining  of  coral 
reefs  to  the  land,  or  by  the  elevation  of  the  sea  bottom,  or  by  vol- 
canic eruption,  or  mountain  growth.  The  Malay  peninsula,  standing 
side  by  side  with  the  island  of  Sumatra,  is  a  mountain  uplift  just 
as  is  Sumatra  itself.  The  Japanese  islands,  and  the  Philippines  to 
tlie  south  of  these,  have  been  lifted  out  of  the  sea  by  mountain  fold- 
ing ;  and  in  time,  if  the  uplift  continues,  they  may  be  joined  to  the 
Kamtchatka  peninsula. 

Tiny  capes  may  also  be  formed  by  wave  erosion ;  and  small  capes, 
as  well  as  great  peninsulas,  like  that  of  Nova  Scotia,  or  of  Labrador, 
may  be  caused  by  the  sinking  of  an  irregular  land.  These  represent 
the  high  places  in  the  ancient  country,  and  the  sinking  has  not  been 
great  enough  to  transform  them  to  islands,  though  by  a  very  small 


SEA  AND  LAKE  SHORES  331 

additional  submergence  Nova  Scotia  would  be  cut  off  from  the  main- 
land, just  as  Newfoundland  now  is. 

Islands  may  be  transformed  to  peninsulas  by  means  of  bars  built 
from  them  to  the  land  by  wave  action  (Fig.  181).  On  the  coast  of  the 
United  States  there  are  many  illustrations  of  this ;  and  sometimes 
the  connection  is  only  made  at  low  tide,  while  in  other  cases,  the  bar 
is  so  high  that  a  road  may  be  built  upon  it. 

Changes  in  Coast  Lines.  —  In  England  the  coast  has 
changed  very  considerably  in  the  past  thousand  years ; 
and  in  America,  during  the  short  period  of  fifty  years 
since  good  maps  of  the  coast  have  been  made,  many 
changes  have  been  noticed.  These  consist  in  a  cutting 
back  of  the  headlands  in  one  place,  the  formation  of  bars 
in  others,  and  the  filling  of  bays  in  still  other  places.  The 
seacoast  is  the  seat  of  very  active  changes,  otherwise  this 
could  not  have  been  seen  by  man. 

Not  only  is  there  this  action  of  the  waves,  but  the  out- 
line of  coasts  is  slowly  changing,  either  through  rising  or 
sinking  (pages  321  and  323).  There  was  a  time  when  the 
New  England  land  extended  many  miles  further  than 
now,  and  when  present  islands  and  capes  were  hilltops, 
and  straits  and  bays  were  dry-land  valleys  between  hills, 
quite  like  those  now  found  in  New  England. 


CHAPTER  XIX 

PLAINS,    PLATEAUS,   AND   MOUNTAINS 

Plains.  —  The  term  plain  refers  to  a  rather  level  stretch 
of  country  of  not  very  great  elevation.  It  is  generally 
somewhat  irregular,  being  crossed  by  streams  which  have 
carved  valleys,  and  its  surface  often  consists  of  a  series 
of  wave-like  undulations.     A  large  plain  presents  the  most 


Fig.  182. 
A  view  on  the  plain  of  the  Everglades  in  southern  Florida. 

monotonous  scenery  to  be  found  in  any  part  of  the  land, 
for  as  far  as  one  may  look  there  is  nothing  but  level 
country.  Where  plains  exist  at  a  considerable  elevation 
above  the  sea,  they  may  be  more  deeply  dissected  by  valleys; 
but  these  higher  plains  are  more  properly  called  plateaus. 

332 


PLAINS,   PLATEAUS,   AND  MOUNTAINS  333 

There  are  many  different  kinds  of  plains.  Bordering  the  coast  of 
Texas,  and  in  fact  most  of  the  states  south  of  New  Jersey,  there  is  a 
narrow  strip  of  level  land  which  is  really  a  part  of  the  old  ocean  bot- 
tom, now  raised  into  the  air,  just  as  plains  would  be  produced  if  the 
continental  shelf  were  elevated.  The  levelness  of  Florida  is  of  the 
same  origin  ;  but  the  delta  and  floodplain  of  the  Mississippi  have  been 
built  by  the  deposit  of  sediment  carried  by  the  river.  Salt-marsh 
plains  have  also  been  built  up,  and  there  are  many  level  stretches 
where  lakes  have  once  existed.  In  North  America  many  small  plains 
and  swamps  are  really  old  lakes;  and  the  great  plain  of  the  Red  River 
valley  of  the  North  represents  an  old  lake  bottom. 

In  addition  to  this,  level  country  may  be  caused  by  denudation,  for 
a  land  may  actually  be  worn  down  until  it  is  nearly  level.  The  plains 
of  the  central  states,  probably  never  very  high  land,  have  been  gradu- 
ally levelled  by  past  denudation;  and  then  much  drift,  left  by  the 
glaciers,  has  been  deposited  upon  their  surface,  so  that  many  parts 
have  been  filled,  making  the  surface  even  more  level  than  it  was 
before  the  Glacial  Period.  Therefore  the  prairies  of  the  Mississippi 
valley  have  a  double  cause  for  their  levelness. 

Even  without  much  denudation,  plains  may  result  if  the  rocks  lie 
in  nearly  horizontal  sheets,  as  they  do  in  the  greater  part  of  this 
country.  With  the  same  climatic  conditions  over  a  large  area,  streams 
will  cut  through  the  rocks,  forming  valleys ;  but  between  these  there 
will  be  level  areas,  because  the  rocks  lie  in  sheets  which  wear  down 
with  the  same  rapidity  in  all  points,  excepting  where  stream  channels 
lie. 

Starting  upon  a  plain,  the  rivers  carve  valleys,  at  first 
narrow  and  steep-sided,  but  in  time  becoming  broader. 
If  the  elevation  is  not  great,  this  does  not  decidedly 
roughen  the  surface ;  but  if  the  region  is  elevated,  the 
valleys  may  become  deep  and  the  country  hilly,  until 
finally  it  loses  the  characteristics  of  a  plain,  as  in  the  case 
of  western  West  Virginia,  Tennessee,  Kentucky,  etc.  If 
time  were  allowed,  the  surface  would  gradually  become 
smooth  again,  and  the  roughened  plain  would  again  become 


334         FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

a  true  plain,  which  would  be  an  old  land  ;  but  for  the  same 
reason  that  old  river  valleys  do  not  exist,  old  plains  are 
not  found. 

Plateaus.  —  A  plateau  is  an  elevated  plain  (Fig.  145) ; 
but  it  also  has  other  features.  Being  more  elevated  than 
a  plain,  the  streams  have  more  power  to  cut,  and  it  is 

REMNANT 
OF  PLAIN 

H 


Fig.  183. 
Diagram  to  illustrate  dissection  of  plain  where  hard  strata  (H)  resist  denuda- 
tion, leaving  rather  flat  hilltops  between  the  river  valleys. 

generally  dissected  by  deep  valleys,  and  even  by  canons. 
Therefore,  although  in  places  the  plateau  is  level,  one 
rarely  has  to  go  far  to  find  it  greatly  roughened,  as  in  the 
plateau  through  which  the  Colorado  River  of  the  west 
cuts  its  canon  (Fig.  145). 

Plateaus  are  generally  formed  by  the  uplift  of  horizon- 
tal beds  of  rock  which  make  the  country  level-topped,  be- 
cause denudation,  in  carving  them,  finds  hard  layers,  which 
being  horizontal,  resist  denudation  everywhere  excepting 
where  the  streams  cut  through  them  (Fig.  183).  As  denu- 
dation proceeds,  the  plateau  may  become  exceedingly  irreg- 
ular and  rough,  as  in  the  case  of  the  Catskill  Mountains, 
and  the  highlands  of  southern  central  and  western  New 
York,  which  are  true,  though  much  dissected  plateaus. 
In  the  course  of  this  denudation  flat-topped  areas  may  be 
left  between  the  streams,  and  in  the  west  the  Spaniards 
have  called  these  mesas,  or  tables.  If  still  smaller  in  area, 
and  rising  steeply  as  a  hill,  these  remnants  of  formerly 
extensive  level  stretches  are  called  buttes  in  the  west 
(Fig.  129). 


PLAINS,   PLATEAUS,   AND  MOUNTAINS  335 

Treeless  Plains.  —  In  this  country  most  of  the  plains  and  plateaus 
are  treeless,  by  far  the  greater  number  of  these  being  so  because  of 
the  dryness  of  the  climate.  This  is  true  of  the  entire  western  plateau 
and  most  of  the  great  plains  west  of  the  Mississippi ;  but  the  cause  of 
the  prairies  of  the  Mississippi  valley  must  be  different,  because  in  this 
region  the  rainfall  is  quite  heavy  enough  for  tree  growth.  The  ex- 
planation of  this  peculiar  absence  of  trees  is  not  entirely  certain. 
Some  have  argued  that  the  soil  is  too  dense,  but  others  believe  that 
the  Indians  have  caused  the  open  prairie,  —  that  they  set  fires  in  con- 
nection with  their  hunt  for  the  buffalo,  and  have  thus  kept  the  land 
clear  of  trees.  The  latter  seems  to  be  the  most  acceptable  explanation, 
for  it  is  known  that  trees  can  grow  in  the  prairie  soil ;  and  moreover 
it  has  been  proved  that  Indians  did  build  fires,  and  that  they  have 
actually  extended  the  area  of  the  prairies  since  white  men  came  into 
the  region.  Both  on  the  plains  and  prairies,  trees  grow  in  the  river 
bottoms  near  the  streams. 

There  are  treeless  plains  in  other  regions:  the  salt  marsh  is  an 
instance;  and  the  low,  level  land  along  the  Texas  coast  is  without 
trees  because  it  is  too  swampy  for  them  to  grow.  The  steppes  of 
Russia  and  the  pampas  and  llanos  of  South  America,  are  great  treeless 
plains. 

Mountains 

Nature  of  Mountains.  —  There  is  very  mucli  difference 
in  the  use  of  the  term  mountain,  but  to  most  it  means  any 
considerable  elevation  above  the  surrounding  country. 
Therefore  in  a  level  region  a  small  hill  is  called  a  moun- 
tain, and  in  mountainous  countries  high  peaks  are  called 
hills.  As  used  in  this  book,  the  term  refers  to  parts  of 
the  earth's  crust  which  have  been  uplifted  as  the  result 
of  folding  or  breaking  of  the  rocks  of  the  crust.  That  is, 
the  strata  have  been  folded  into  waves,  as  we  might  fold 
the  leaves  of  this  book.  In  plateaus  the  rock  layers  re- 
main nearly  horizontal,  as  they  were  deposited ;  but  when 
raised  into  mountains  they  have  been  inclined. 


336 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


This  folding  of  the  rocks  has  occurred  in  various  parts 
of  the  earth,  the  most  notable  cases  in  this  country  being 
the  Appalachians  in  the  east,  and  the  several  mountain 
ranges  occupying  the  western  part  of  the  United  States. 
In  these  places  the  rocks  have  been  tilted,  either  by  folding 
or  faulting,  so  that  they  rise  above  the  general  level, 
sometimes  to  heights  of  5000  or  10,000  feet.  A  series  of 
such  folds  forms  a  system^  such  as  the  Appalachians,  which 
extend  from  Alabama  to  New  York,  or  the  Rockies,  reach- 


CV/S      CO     cws 


Fig.  184. 

Sections  through  portions  of  the  Appalachians,  showing  folded  roclcs  and  faults. 
Former  extension  of  strata  indicated  by  dotted  lines.  Letters  identify 
layers. 

ing  from  Mexico  into  Canada.  Two  or  more  systems 
closely  associated,  such  as  the  Rockies,  Basin  Ranges, 
Sierra  Nevada,  and  Coast  Ranges,  constitute  a  Cordil- 
lera. 

In  each  system  there  are  parts  known  as  ranges^  which 
consist  of  uplifted  portions  side  by  side  and  separated  by 
valleys ;  and  in  each  range  there  may  be  single  ridges 
(Plate  20).  The  characteristic  feature  of  each  of  these  is 
that  the  rocks  are  tilted,  so  that  the  length  of  the  elevation 
is  greater  than  the  width.  But  the  most  striking  feature 
among  high  mountains  is  the  peak  (Fig.  185),  a  lofty 
elevation  whose  length  and  width  do  not  greatly  differ, 


PLAINS,   PLATEAUS,  AND  MOUNTAINS 


337 


but  which  rises  above  the  surrounding  region,  sometimes 
to  a  great  height.  It  is  the  mountain  peak  which  people 
have  in  mind  when  they  name  a  hill,  a  mountain.  Really 
these  peaks  are  true  hills  of  great  size  and  height  among 
mountains ;  and  it  is  not  necessary  that  the  rocks  of  which 
they  are  made  shall  be  folded. 


Fig.  185. 
Grandfather  Mountain ;  a  peak  in  the  Blue  Ridge  of  North  Carolina. 

Development  of  a  Mountain  System.  —  In  order  to  under- 
stand the  origin  of  the  features  of  mountains,  perhaps  the 
best  way  to  proceed  is  to  imagine  that  we  can  trace  the 
growth  of  a  mountain  system.  Let  us  suppose  that  it 
starts  in  the  sea,  where  for  some  reason  the  bed  is  being 
raised  and  the  layers  of  the  bottom  are  being  folded. 
Gradually  the  bed  rises,  with  little  other  change  than  the 
folding  (or  perhaps  faulting)  and  uplifting,  though  proba- 
bly volcanoes  pour  forth  lava  at  various  points  along  the 
crests  of  the  rising  ridges.  Such  a  mountain  range  exists 
in  the  sea  along  the  line  where  the  Hawaiian  Islands  rise 


338  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

above  the  surface,  and  this  range  extends  not  less  than 
1500  miles. 

As  soon  as  the  range  rises  into  the  air,  a  new  chapter 
begins.  Formerly,  as  the  folds  gradually  rose,  there  was 
nothing  to  check  the  increasing  elevation ;  but  as  soon  as 
they  reach  the  air,  the  agents  of  denudation  commence 
their  work  of  sculpturing  and  destruction,  rains  fall,  winds 
blow,  rocks  decay  in  the  weather,  rivers  gather  on  the  land, 
and  perchance  tlie  ocean  waves  beat  against  the  margin. 
So  the  folds  no  longer  rise  uninterruptedly,  but  their 
elevation  is  ineffectually  opposed,  and  they  continue  to 
rise  with  battered  and  scarred  surfaces,  never  reaching  the 
height  to  wliich  they  would  have  risen  had  denudation 
been  debarred.  Tliey  begin  to  be  sculptured,  and  the 
hard  rocks  stand  up  because  the  softer  ones  are  worn 
away  more  rapidly. 

Such  a  stage  has  been  reached  by  the  Japanese  Islands, 
which  are  even  now  rising  mountains.  They  represent  a 
mountain  system  with  several  ranges,^  but  denudation  has 
cut  into  the  ranges,  and  finding  hard  layers  of  rock  in  the 
form  of  inclined  sheets,  has  carved  out  ridges  where  the 
hard  layers  exist.  Because  the  rocks  were  folded  as  sheets, 
the  ridges  are  nearly  parallel  ;2  and  wherever  denudation 
finds  such  hard  layers,  ridges  may  be  formed,  as  has  hap- 
pened in  the  Appalachians  (Fig.  143),  which  are  old  moun- 
tains that  have  been  exposed  to  the  air  for  a  long  time. 
But  if  there  exists  a  more  massive  rock,  like  granite,  not 
standing  in  a  layer,  it  also  will  resist  denudation  and  stand 

^  As  we  may  fold  paper  into  two  or  three  folds,  each  representing  a 
range,  and  the  whole  a  system. 

2  As  our  paper  mountain  would  be  if  we  should  take  a  pair  of  shears 
and  clip  off  the  tops  of  the  folds. 


PLAINS,  PLATEAUS,  AND  MOUNTAINS  839 

up,  thus  forming  not  a  ridge,  but  a  peak  (Fig.  185).  A 
very  large  number  of  well-known  peaks  in  the  world  ^  rise 
above  the  surrounding  folds,  because  some  such  hard  rock 
as  granite  has  resisted  denudation  better  than  the  softer 
ones  that  surround  it.  Had  the  durable  rock  existed  as  a 
la^er,  a  ridge  would  have  been  produced,  though  perhaps 


Fig.  186. 
Mt.  Moran,  in  Teton  Mountains. 

one  which  has  been  carved  into  many  peaks  along  the  line 
of  the  ridge,  as  seen  in  the  Teton  Mountains  of  Wyoming, 
and  others  in  the  west. 

Carrying  the  development  of  the  mountain  system  still 
further,  it  may  rise  and  become  a  part  of  the  continent,  as 
the  Japanese  Islands  will  if  they  continue  to  be  elevated,  and 
as  the  Coast  ranges  of  California  already  have.  Between 
them  and  the  mainland,  there  is  at  first  a  partly  enclosed 

1  Such  as  the  peaks  of  the  White  Mountains  of  New  Hampshire,  the 
Adirondacks,  Pike's  Peak,  Mt.  Everest  in  the  Himalayas,  the  Matter- 
horn,  and  otlier  Swiss  peaks,  as  well  as  hundreds  of  others. 


340  FIRST  BOOK  OF  PHYSICAL  GEOGRAPHY 

sea,  then  as  the  outlet  rises  above  sea  level,  great  enclosed 
lakes  outflowing  to  the  sea,  and  then  perhaps  a  great 
valley.^  Finally,  as  the  elevation  continues,  the  valley 
may  become  a  plateau,  or  perhaps  an  enclosed  basin,  like 
the  Great  Basin  between  the  Sierra  Nevada  and  the 
Rockies,  into  which  rivers  flow  without  passing  into  the 
sea,  because  their  water  is  evaporated  by  the  dry  air,  which 
has  been  robbed  of  its  moisture. 

By  the  growth  of  the  mountains  not  only  may  great 
basins  be  enclosed  between  the  systems,  but  other  valleys 
of  smaller  size  may  form  between  the  ranges,  while  rivers 
carve  still  others  on  the  mountain  sides,  across  the  ranges 
(Fig.  143),  and  between  the  ridges.  The  valleys  which 
the  rivers  carve  are  deep  canons  with  steep  and  rocky 
sides,  because  the  water  courses  down  them  with  great 
velocity,  and  therefore  can  dig  rapidly  (Fig.  184). 

The  Destruction  of  Mountains.  —  Although  mountains 
rise  slowly,  so  long  as  they  grow  their  elevation  is  usually 
more  rapid  than  the  downcutting  by  denudation ;  but  the 
streams  have  such  a  slope  that  their  valleys  are  deeply 
cut,  and  differences  of  rock  texture  are  brougni  into  very 
sharp  relief.  It  is  not  merely  because  of  the  slope  of  the 
streams,  but  also  because  the  peaks  rise  into  the  higher 
regions  of  the  atmosphere,  where  winds  are  fierce  and  frost 
action  powerful,  so  that  weathering  is  rapid  (Fig.  127). 
Moreover,  the  high  mountain  tops  rise  above  the  timber 
line  (Figs.  85  and  187),  and  are  therefore  not  protected 
from  the  weather.  Hence  it  is  that  a  young,  and  particu- 
larly a  growing  mountain  is  carved  into  very  rugged 
forms.    Just  as  cafions  are  characteristic  of  young  streams, 

1  Like  the  Sacramento  valley  of  California,  which  lies  between  the 
Coast  Pianerra  and  Sierra  Nevada. 


PLAINS,  PLATEAUS,  AND  MOUNTAINS 


341 


so  rugged  mountains,  like  the  Rockies,  Andes,  Alps,  and 
Himalayas,  are  also  young. 

But  in  the  course  of  time  there  comes  a  period  when 
the  mountain-building  forces  cease  to  cause  the  ridge  to  rise, 
and  then  denudation  has  full  sway,  and  the  ranges  slowly 
melt  down.  The  Appalachians,  consisting  of  low  ridges 
strikingly  different  from  the  Alpine  ranges,  have  for  a  long 


Fig.  187. 
Near  the  timber  line,  Gallatin  Range,  Montana. 

time  been  subjected  to  destructive  agents,  until  they  have 
lost  their  ancient  elevation  and  irregularity.  In  fact  denu- 
dation may  go  further  than  this,  and  lofty  ranges  be  re- 
duced to  low  hills.  Nova  Scotia,  all  of  New  England,  and 
the  very  sites  of  the  cities  of  New  York,  Philadelphia,  Bal- 
timore, Washington,  and  Richmond,  are  all  reduced  moun- 
tains which  once  rose  to  heights  rivalling  those  of  the  Rockies 
and  the  Alps.  We  know  this  because  the  rock  layers  are 
folded  in  such  a  way  that  if  they  were  continued,  as  they 
once  must  have  been,  they  would  rise  thousands  of  feet 
into  the  air  (Fig.  181).     Tlicrefore,  such  is  the  length  of 


342  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

time  since  the  earth  was  young,  and  so  powerful  are  the 
agents  of  denudation,  that  if  they  have  plenty  of  time 
in  which  to  work,  even  mountain  ranges  may  be  reduced 
to  low  hills. 

Other  Kinds  of  Mountains.  —  Wliile  only  elevations  due  to  folding 
or  faulting,  or  true  mountain  ranges,  have  been  considered  here,  it 
must  be  said  that  there  are  true  mountainous  elevations  which  may 
be  formed  without  rock  folding.  The  real  mountain  peaks  are  caused 
by  the  sculpturing  of  rocks  among  ranges;  but  wherever  unusually 
high  areas  have  been  formed  by  the  unequal  carving  of  the  strata, 
mountain  peaks  may  result,  even  though  not  situated  in  regions  of 
folded  rock.  Many  of  the  buttes  of  the  west  are  high  hills  or  low 
peaks,  and  therefore  properly  called  mountain  peaks. 

Indeed,  ranges  of  considerable  extent  may  be  formed  where  un- 
usually durable  rocks  have  resisted  destruction  better  than  the  neigh- 
boring land ;  and  this  is  the  case  in  the  Catskills,  where  the  strata 
are  not  folded,  but  where  hard  sandstone  occurs,  which  is  much  more 
durable  than  the  suri-ounding  layers.  They  therefore  stand  higher, 
and  have  been  carved  into  very  irregular  form,  so  that,  though  they 
are  really  a  very  much  sculptured  plateau,  they  closely  resemble  a 
true  mountain  range. 

The  Cause  of  Mountains.  —  The  origin  of  mountains 
leads  us  to  a  question  about  which  very  little  is  known. 
Since  it  is  not  possible  to  state  even  the  most  important 
suggested  explanations,  nor  to  discuss  them  fully,  it  will 
be  well  to  state  but  one,  the  contraction  theory^  which  has 
found  most  favor,  though  it  must  be  said  that  it  has  not 
been  proved.  We  know  that  the  heat  of  the  earth  in- 
creases with  the  depth,  and  it  is  believed  that  the  interior 
is  highly  heated.  If  this  is  so,  the  earth  is  cooling;  for 
the  heat  is  escaping  into  space  just  as  certainly  as  it  is 
from  a  stove  into  a  room.  A  hot  body  is  larger  than  it 
would  be  if  cool,  and  therefore,  as  it  cools,  it  becomes 
smalleio 


PLAINS,   PLATEAUS,   AND  MOUNTAINS  343 

So  the  theory  is,  that  the  earth,  a  hot  body  within, 
surrounded  by  a  cold,  solid  outer  crust,  is  cooling,  and 
hence  slowly  shrinking.  The  interior  is  therefore  con- 
stantly becoming  smaller ;  but  the  solid  crust,  already 
cool,  is  not  losing  size,  and  therefore  it  must  either  be 
separated  from  the  interior  or  else  sink  down  upon  it, 
as  the  core  becomes  smaller.  If  this  is  done,  the  only 
way  in  which  it  can  fit  the  shrinking  central  part  is  by 
crumpling,  as  the  skin  of  an  apple  does  when  this  dries, 
losing  water  from  the  inner  pulp  and  therefore  becoming 
smaller.  This  supposed  loss  of  bulk,  or  contraction  of  the 
earth's  interior,  is  a  very  slow  process,  and  therefore  the 
uplifting  of  mountains  will  also  be  slow.  The  elevation 
of  the  crust  will  occur  along  lines  which  for  some  reason 
are  weaker  than  other  places. 

Really,  contraction  causes  the  earth's  surface  to  slowly 
settle,  and  this  sinking  is  evidently  occurring  over  most  of 
the  sea  bottom ;  but  locally,  along  mountain  ranges,  and  in 
the  continents,  portions  are  rising,  for  the  crust  has  a 
greater  diameter  than  the  shrinking  interior,  which  is  ever 
becoming  smaller;  and  hence,  while  the  greater  part  sinks, 
some  must  rise.^  To  the  contraction  theory  there  are  some 
objections,  though  none  that  seem  fatal  to  it  as  an  explana- 
tion ;  and  it  noAV  stands  as  the  best  attempt  so  far  made  to 
explain  the  fact,  which  all  know  so  well,  but  whose  cause 
is  somewhere  in  the  earth  be3"ond  the  reach  of  human  vision. 
Not  being  able  to  see  and  hence  thoroughly  understand 
what  is  going  on  below  the  surface,  we  can  only  reason 
upon  the  basis  of  the  facts  which  we  already  possess. 

1  This  can  be  proved  experimentally  by  taking  a  ball  and  attempting 
to  make  a  flannel  cover,  a  little  larger  than  the  ball,  fit  upon  its  surface. 
In  doing  this  there  will  be  some  ridges  of  cloth. 


CHAPTER  XX 

VOLCANOES,  EARTHQUAKES,  AND  GEYSERS 

Volcanoes 

Birth  of  a  Volcano.  — In  the  year  1831,  in  the  Mediter- 
ranean south  of  Sicily,  quite  without  warning,  there  rose 
out  of  the  sea  a  great  volume  of  steam,  bearing  in  it  red- 
hot  cinders,  which  falling  back,  built  a  shoal  in  the  sea, 
which  shortly  rose  as  an  island.  A  volcano  was  born,  and 
its  birth  was  announced  by  a  roar  and  commotion  of  the 
water.  In  a  short  time  quiet  again  reigned,  and  in  a  few 
years  Graham's  Island  had  disappeared  before  the  attack 
of  the  waves,  and  now  no  land  exists  to  mark  its  site. 

The  new  island  was  low  (being  about  200  feet  high, 
and  3  miles  in  circumference),  and  was  built  entirely  of 
loose  fragments  of  volcanic  ash,  light  in  weight  because 
pierced  by  innumerable  tiny  cavities,  quite  like  pumice. 
It  had  risen  through  the  crust  as  liquid  molten  rock, 
driven  upward  by  the  steam,  which  expanding  in  the 
lava,  had  blown  it  full  of  holes.^  The  steam,  escaping 
as  it  does  from  an  engine,  carried  the  ash  high  into  the 
air,  until  spreading  out  above,  it  was  brought  down  by 
gravity,  falling  at  one  side  of  the  vent  through  which  it 
had  escaped.     This  vent  the  steam  kept   clear,  so   that 

1  As  steam  rises  through  oatmeal,  or  as  gas  makes  porous  the  bread 
that  is  becoming  solid  in  baking. 

344 


VOLCANOES  346 

when  the  eruption  ceased  there  was  a  cavity  or  crater 
there.  Most  of  the  ash,  and  especially  the  heavier  pieces 
that  settle  more  quickly,  fell  near  the  outlet  on  all  sides 
of  it,  forming  a  cone,  which  was  nearly  as  steep  as  the 
angle  at  which  loose  fragments  of  rock  will  rest  in  the 
air.i 

This  newly  born  volcano,  which  died  after  a  single  gasp, 
illustrates  perfectly  a  typical  volcano,  though  it  rose  only 
about  200  feet,  while  the  elevation  of  most  cones  is 
measured  in  thousands  of  feet.  Had  there  been  another 
eruption,  the  size  of  the  cone  would  have  been  increased, 
and  in  time  a  large  volcano  would  have  been  built;  but 
during  its  history,  perhaps  with  a  life  of  tens  of  thousands 
of  years,  there  would  have  been  many  changes,  for  volcanic 
action  is  very  capricious.  During  long  periods  it  might 
have  been  quiet,  then  perhaps  a  violent  eruption  might 
have  partly  destroyed  the  cone,  sending  it  into  the  air; 
and  first  ash  might  have  been  erupted  from  the  vent,  and 
later  lava.  Also  throughout  its  history,  denudation,  the 
enemy  of  the  land,  would  have  attacked  it,  removing  some 
of  the  materials  out  of  which  it  was  built.  To  understand 
what  might  have  happened  during  its  history,  let  us  look  at 
what  has  happened  to  some  of  the  volcanoes  of  the  earth. 

Vesuvius.  —  When  the  Italian  peninsula  was  first  visited 
from  the  east,  a  lofty  conical  mountain  rose  above  tlie  Bay 
of  Naples.     As  time  passed,  towns  were  built  at  its  base 

1  An  experiment  illustrating  this  can  be  shown  by  having  a  U-shaped 
tube,  containing  sand,  with  one  end  rising  through  a  piece  of  cardboard 
partly  resting  on  a  table.  Upon  blowing  through  the  tube  the  sand  rises 
in  the  air  and  builds  a  cone  with  a  crater.  Care  must  be  taken  not  to  put 
too  much  sand  in  the  tube  ;  and  in  order  to  build  a  high  cone,  sand  must 
be  put  into  the  tube  several  different  times. 


346  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

and  upon  its  sides.  It  had  the  form  of  a  volcano  with  a 
crater  in  the  centre,  but  it  did  not  erupt.  Apparently, 
therefore,  it  was  a  dead  or  extinct  cone  open  only  to  de- 
struction from  denudation.  For  centuries  this  condition 
lasted ;  but  the  volcano  was  not  extinct,  it  was  only  sleep- 
ing or  dormant.  About  the  year  79  a.d.  the  Bay  of  Naples 
was  visited  by  earthquake  shocks.  Monte  Somma,  the 
ancestor  of  Vesuvius,  was  preparing  for  a  terrific  eruption, 
and  during  the  year  79  an  outbreak  occurred,  so  violent, 
that  when  it  ceased,  the  cone  was  changed  in  form.  Part 
of  the  old  rim  of  Monte  Somma  had  disappeared,  being 
blown  into  the  air,  while  a  new  and  smaller  cone  had  been 
built  amid  the  rains. 

Monte  Somma  slept  no  more,  and  after  a  rest  of  man}^ 
centuries  tlie  active  Vesuvius  was  born.  Ash  rose  thou- 
sands of  feet  in  the  air,  and  spreading  out,  foimed  a  cloud 
so  dense  that  the  day  became  as  dark  as  night.  The  ash 
fell  upon  the  flanks  of  the  mountains  and  upon  the  neigh- 
boring lowland.  People  fled  before  the  shower  of  hot  rock. 
Homes  and  towns  were  abandoned,  and  when  the  erup- 
tion had  ceased,  the  sites  of  cities,  villages,  and  farms  were 
covered  by  a  great  barren  stretch  of  pumice  and  ash.  For 
centuries  these  were  buried,  and  even  now,  no  doubt,  scores 
of  towns  are  entombed  beneath  the  products  of  this  erup- 
tion. Two  of  them,  Pompeii  and  Herculaneum,  have  been 
discovered  and  extensively  excavated,  showing  the  dwell- 
ings of  the  people  who  were  driven  from  them  more  than 
eighteen  centuries  ago  (Fig.  188). 

Since  that  terrible  eruption  of  79,  Vesuvius  has  had 
periods  of  quiet;  but  every  now  and  then  an  outbreak  has 
occurred,  and  the  mountain  is  still,  at  the  present  day,  an 
active  volcano.     There  have  been  rests  of  many  years,  at 


VOLCANOES 


347 


other  times  frequent  eruptions.  Sometimes  the  outbreaks 
have  been  violent,  again  relatively  quiet;  but  at  no  time 
has  there  been  such  a  destructive  explosion  as  that  which 
gave  modern  Vesuvius  its  birth.  Often  the  material 
erupted  has  been  ash ;  but  at  other  times  liquid  rock  has 
welled  out,  and  flowing  down  the  mountain  side,  has  cooled 
to  form  solid  lava.     Therefore  this  volcano  has  sent  out 


Fia.  188. 
A  portion  of  Pompeii  excavated  from  beneath  the  ash.    Vesuvius  in  the  back- 
ground.   A  part  of  the  old  rim  of  Monte  Somma  on  the  right. 

both  ash  and  lava,  so  that  it  differs  from  Graham's  Island, 
which  had  but  one  eruption,  and  that  of  ash. 

Krakatoa. —  In  the  late  summer  of  1883,  the  sailors  in 
the  Straits  of  Sunda  saw  a  great  cloud  rising  above  a  small 
island,  and  this,  U[)()U  spreading  out,  obscured  the  sun, 
while  ash  fell  from  the  air.     Upon  the  neighboring  land 


348  FIRST  BOOK  OF  PHYSICAL   GEOGIiAPnT 

the  same  was  seen,  and  the  ground  was  shaken,  while  upon 
the  low  coasts  a  great  water  wave  rushed,  destroying  thou- 
sands of  lives.  Krakatoa,  which  had  not  been  in  eruption 
during  this  century,  had  again  broken  forth,  with  the  most 
terrific  explosion  tliat  man  has  recorded.  Ash  rose  miles 
in  the  air,  and  spreading  out,  fell  on  the  surrounding  land 
and  water,  and  for  awhile  it  was  so  thick  upon  the  surface 
of  the  sea,  in  the  Straits  of  Sunda,^  that  the  progress  of 


Fig.  189. 
Tlie  volcauo  of  Mauua  Loa,  Hawaiian  Islands. 

vessels  was  impeded.  So  high  did  it  rise  that  the  lighter 
ash,  floating  about  by  the  upper  winds,  staid  suspended  in 
the  air  for  months,  some  of  it  falling  in  America  and 
Europe.  A  great  water  wave,  generated  by  the  explosion, 
crossed  the  Pacific  to  the  California  coast,  and  it  was  ob- 
served on  the  shores  of  Africa  and  Australia. 

When  the  eruption  had  ceased  it  was  found  that  Kraka- 
toa had  been  split  into  two  parts,  one  of  which  had  disap- 
peared into  the  air,  leaving  ocean  water  where  there  had 
1  For  volcanic  ash  and  pumice  \fill  float  in  water. 


VOLCANOES 


349 


been  dry  land.  The  part  of  the  island  that  remained  was 
covered  with  a  deep  coating  of  ash,  and  not  a  living  thing 
was  left,  neither  plant  nor  animal.  Since  then  there  have 
been  no  more  eruptions ;  and  now  Krakatoa  is  either  dor- 
mant or  extinct,  but  which,  cannot  be  told  for  centuries. 
This  eruption  may  have  been  the  death  struggle  of  the 
once  mighty  cone,  or  it  may  have  been  but  a  temporary 
awakening,  after  a  long  rest. 


Fig.  190. 
The  lava  floor  in  the  crater  of  Kilauea. 


The  Hawaiian  Volcanoes.  —  A  series  of  eight  islands  lie 
in  a  chain  in  the  mid-Pacific,  and  all  of  them  have  been 
built  by  volcanic  eruptions.  Most  are  now  extinct  and 
are  rapidly  disappearing  before  the  attacks  of  wind, 
weather,  rivers,  and  waves ;  but  upon  the  largest,  Hawaii, 
there  are  several  craters,  one  Kilauea,  another  Mauna  Loa 
(Fig.  189),  and  a  third  Mauna  Kea.  The  latter  rises 
13,805   feet  above  the  s^a,  Ma-una  Loa  13,G75  feet,  and 


350 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


Kilauea  about  9000  feet.     The  two  latter  are  now  active 
and  have  been  well  studied. 

One  may  ascend  to  the  top  of  the  immense  crater  of  one 
of  these  and  look  down  upon  a  lake  of  liquid  rock,  through, 
which  jets  of  steam  rise,  occasionally  throwing  bits  of  lava 
into  the  air.     There  is  no  danger  in  this  journey,  which  is 

constantly  being 
made  by  tourists, 
while  a  house  is 
built  for  their  ac- 
commodation on 
the  margin  of  the 
Kilauea  crater. 
Once  in  a  while, 
however,  on  an 
average  of  once  in 
about  seven  years, 
an  eruption  occurs ; 
but  it  is  not  at  all 
like  those  just  de- 
scribed. There  is 
no  ash,  but  a  flow 
of  lava,  not  from 
the  crater,  but  out 
of  the  side  of  the 
cone,  through 
which  the  molten  rock  escapes  by  a  fissure  which  had 
broken  open  the  side  of  the  mountain.  From  this  the 
lava  flows  down  the  mountain  side,  sometimes  reaching 
the  sea,  and  perhaps  passing  30  or  40  miles  before  coming 
to  an  end.  At  first  it  flows  rapidly,  a  glowing  stream  of 
liquid  rock ;  but  soon,  becoming  cooled  in  the  air,  a  crust 


Fig.  191. 

An  eruption  of  a  tiny  volcano  in  the  Mediter- 
ranean. 


VOLCANOES  351 

forms  on  the  top,  under  which  the  molten  lava  is  enclosed. 
Then  it  flows  less  rapidly  and  finally  barely  creeps,  slowly 
advancing  for  weeks,  until  at  last,  when  the  force  from 
behind  is  exhausted,  it  stops,  perhaps  at  the  very  outskirts 
of  some  town  which  it  has  threatened  to  destroy.  Nearly 
all  the  eruptions  of  these  volcanoes  have  been  of  this 
nature,  —  flows  of  black  lava,  known  as  basalt. 

Other  Volcanoes.  —  From  the  hundreds  of  cones  in  the  world  ^\e 
might  select  other  instances ;  but  these  that  have  been  given,  f urnisli 
illustrations  of  the  chief  differences.  Some,  like  Fusiyama  in  Japan, 
and  many  in  the  Andes,  always  erupt  ash ;  others,  like  the  Hawaiian 
cones,  practically  always  emit  lava;  but  most,  like  Vesuvius,  ^tna, 
and  the  volcanoes  of  Iceland,  now  erupt  ash  and  now  lava.  Some, 
like  the  tiny  volcanoes  of  the  Lipari  Islands  in  the  Mediterranean, 
have  toy  eruptions,  so  moderate  that  they  may  be  witnessed  from  a 
ship,  near  by,  without  danger ;  and  from  tliese  there  is  every  gradation, 
to  those  terrible  eruptions  of  Vesuvius  and  Krakatoa  just  described, 
and  the  frightfully  destructive  outbursts  of  the  Icelandic  volcanoes. 
While  in  some  cases  the  outbreaks  are  frequent,  in  others  they  come 
at  irregular  intervals,  perhaps  centuries  apart.  Even  the  tiniest 
eruption  is  an  impressive  scene ;  but  a  violent  one  is  the  most  awe- 
inspiring  phenomenon  which  nature  presents  upon  the  earth's  surface. 

Materials  Erupted.  —  In  volume  and  importance,  steam  is  the 
greatest  of  volcanic  products.  Often  in  the  quiet  eruptions  of  Kilauea, 
great  banks  of  steam  rise  from  the  lava;  and  in  great  eruptions  so 
much  rises,  that  reaching  thousands  of  feet  into  the  air,  it  condenses 
into  drops,  and  falling  back  to  the  ground,  produces  deluging  rains, 
so  heavy  that  torrents  rush  down  the  sides  of  the  cone,  adding  to  the 
destruction  caused  by  the  other  products.  Perchance  falling  upon 
loose  ash,  it  washes  this  down  with  it  in  such  quantities  that  a 
destructive  torrent  of  mud  passes  on,  overwhelming  everything  in  its 
path.  Ilerculaneum  on  the  flanks  of  Vesuvius  was  buried  beneath 
such  a  mudjiow. 

Other  gases  than  steam  arise,  but  they  are  of  much  less  impor- 
tance. Sometimes  the  lava  wells  out  quietly  as  a  flow,  and  there  is 
always  lava  in  the  tube  of  a  volcano.     However,  when  for  any  reason 


352  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

the  steam  has  the  power,  the  lava  is  blown  into  shreds  and  bits,  and 
upon  cooling,  forms  ash  and  pumice.  These  vary  in  size  from  mere 
bits  of  dust  to  huge  blocks  of  rock.  In  nearly  every  case  the  ash  is 
merely  lava  which  the  steam  has  blown  out,  though  sometimes,  when 
a  mountain  side  like  that  of  Krakatoa  is  blown  into  the  air,  it  is 
apparently  made  up  of  the  broken  fragments  of  the  mountain,  like  the 
bits  of  iron  which  are  thrown  out  during  a  violent  boiler  explosion. 

Form  of  the  Cone.  —  A  typical  volcano  is  a  cone  with 
a  crater  in  the  centre.  Sometimes  the  perfection  of  this 
has  been  destroyed  by  a  violent  explosion,  like  that  Avhich 
split  Krakatoa,  and  that  of  79  A.D.,  which  partly  destroyed 
Monte  Somma.  Some  cones  are  very  low,  if  the  eruptions 
have  been  few,  while  others  are  of  great  size.  There  is 
also  a  difference  in  the  shape  of  the  peak.  For  instance, 
Mauna  Loa,  though  rising  above  the  sea  to  tlie  height  of 
13,G75  feet,  does  not  rise  steeply  (Fig.  189).  Its  diameter 
at  the  base  is  great,  and  hence  it  is  a  moderately  sloping, 
but  very  high  cone.  This  is  because  the  material  of  which 
it  is  built  is  lava,  which  flowing  aAvay  from  the  place  of 
exit,  extends  over  a  score  or  two  of  miles,  first  as  a  liquid, 
then  as  a  more  viscous  body.  Fusiyama  in  Japan,  and 
many  of  the  South  American  volcanoes,  are  narrow  at  the 
base  but  very  steep.  These  are  made  of  volcanic  ash  which 
has  settled  near  the  crater,  taking  an  angle  as  steep  as 
loose  fragments  can  maintain  in  the  air,  just  as  a, heap  of 
dirt  will  when  dumped  from  a  wagon.  Those  that  are 
made  partly  of  ash  and  partly  of  lava  will  be  less  steep, 
and  these  are  the  most  common  of  volcanoes,  and  are 
therefore  more  typical  than  either  the  steep  Fusiyama  or 
the  great  mound  of  Mauna  Loa. 

There  is  another  point,  too :  when  a  volcano  erupts  ash 
some  is  lost,  for  the  winds  carry  it  away ;  but  in  the  lava 


VOLCANOES 


353 


eruption  all  remains  within  a  score  or  two  of  miles  of 
the  cone.  Hence  a  much  larger  peak  will  be  made  by  the 
same  number  of  lava  eruptions  than  of  ash,  provided  the 
quantity  sent  out  is  the  same.  Therefore  the  hulk  of 
Tock  in  such  a  cone  as  Mauna  Loa  is  many  times  as  great 
as  that  of  some  of  the  ash  emptors. 

Extinct  Volcanoes.  —  Not  only  do  volcanoes  erupt,  be- 
come dormant,  and  then  active  again,  but  sometime  in  its 
history  every  volcano  will  die,  as  certainly  as  every  animal 


Popocatapetl,  Mexico. 


Fig.  192. 

A  dormant  or  else  extinct  volcano  rising  above  the 
plateau. 


and  plant  will.  Of  extinct  volcanoes  we  have  thousands 
of  instances  in  the  world,  but  in  no  part  of  the  earth  are 
they  more  numerous  than  among  the  Cordilleras  of  the 
western  part  of  this  country  and  Mexico.  Some  have 
ceased  erupting  for  so  short  a  period  that  it  cannot  be 
certainly  stated  that  they  are  more  than  dormant  (Fig. 
192).     It  need  surprise  no  one  to  hear  that  a  volcano  in 


854 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHT 


the  west  has  again  burst  forth  into  activity.  Near  some 
there  are  lava  flows  that  have  certainly  not  been  exposed 
to  the  air  for  a  full  century. 

Cones  forming  under  the  sea^  rise  without  being  attacked 
by  the  forces  of  destruction ;  but  all  through  their  history 
volcanoes  that  rise  in  the  air  are  subject  to  the  attack  of 
the  agents  of  denudation.  The  work  of  these  is  not  so 
rapid  as  the  supply  of  lava  or  ash,  and  so  the  cones  rise ; 
but  when  these  supplies  are  cut  off,  they  slowly  melt  away. 


Fig.  193. 
Volcanic  necks  or  plugs,  remnants  of  volcanoes,  on  the  plateau  of  New  Mexico 


Among  the  Hawaiian  Islands,  and  elsewhere  in  the  Pa- 
cific, there  are  cones  in  all  stages  of  destruction,  and  the 
same  is  true  among  the  plateaus  and  mountains  of  the 
west.  First  the  crater  is  breached  and  gullied  (Fig.  192), 
then  the  cone  form  disappears,  and  finally  only  the  hard 
core  of  solid  lava  in  the  tube  or  neck  stands  up.     All  of 

1  It  seems  certain  that  there  are  active  craters  in  places  in  the  sea,  par- 
ticularly on  the  Asiatic  coast. 


VOLCANOES 


355 


the  cone,  the  ash  and  the  lava  flows,  have  now  disappeared. 
There  are  hundreds  of  these  remnants  in  the  west  (Fig. 
193). 

In  past  ages  volcanic  cones  existed  in  New  England,  especially  in 
the  Connecticut  valley,  and  from  there  along  the  Atlantic  coast  at 
least  as  far  as  North  Carolina.  Some  of  the  ancient  lava  flows  still 
remain  buried  beneath  other  rocks,  but  the  cones  have  long  since  dis- 
appeared ;  and  in  the  east  there  have  been  no  volcanoes  in  times  suffi- 
ciently recent  to  have  left  even  a  part  of  a  cone. 


Fig.  194. 

Diagrammatic  sketch  to  show  (by  dots)  distribution  of  volcanoes. 

Distribution  of  Volcanoes. — -At  present  volcanic  cones 
are  most  abundant  along  the  borders  of  the  Pacific  Ocean, 
though  there  are  some  elsewhere.  In  all  cases  they  are 
either  in  or  near  the  sea;  and  generally,  if  not  always, 
they  are  associated  with  mountains  that  are  growing. 
Those  that  are  extinct,  like  the  partly  destroyed  cones  of 
the  west,  are  found  among  mountains  that  have  ceased 
rising;  and  it  seems  certain  that  as  soon  as  mountains 
cease  their  growth,  the  volcanoes  that  exist  among  them 


356  FIBST  BOOK  OF  PHYSICAL    GEOGRAPHY 

die  out.  Very  often,  as  in  the  Hawaiian  and  Japanese 
islands,  the  cones  extend  in  chains  along  lines. 

Cause  of  Volcanoes.  —  Some  action  or  condition  in  the  earth  melts 

the  rocks  in  places.  Some  believe  that  the  roots  of  volcanoes  reach 
through  the  crust  into  the  zone  w^here  the  heat  is  high  enough  to  melt 
strata,  and  certainly  there  is  such  a  zone,  if  the  temperature  of  the 
earth  increases  at  the  same  rate  as  it  does  in  the  part  so  far  explored. 
Others  think  that  the  movement  of  rocks  in  mountain  folding  causes 
melting  by  the  heat  of  friction  of  the  particles  moving  over  one 
another,  as  we  may  warm  two  pieces  of  rock  by  rubbing  them  to- 
gether. Since  this  mountain  folding  is  probably  due  to  the  heat 
within  the  earth,  in  either  case  the  Jirst  cause  for  volcanoes  is  this 
heated  condition;  though  in  the  second  explanation,  it  is  thought 
that  the  melting  is  a  secondary  result  of  this. 

Whichever  of  these  is  correct,  the  expelling  force  of  the  lava  is 
steam,  so  that  this  is  the  immediate  cause  for  the  eruption,  as  it  is  in 
tlie  explosion  of  a  boiler.  The  reason  for  the  association  of  growing 
mountains  with  volcanoes,  may  be  the  melting  of  rocks,  or  it  may  be 
the  squeezing  of  the  melted  rocks  up  to  places  where  they  can  escape. 
Whichever  is  the  true  explanation,  the  folding  of  the  strata  causes 
breaks,  or  fissures  and  faults,  through  which  the  lava  may  escape. 
This  accounts  for  their  arrangement  along  lines. 

Explanation  of  the  Differences  in  Volcanoes.  —  When  the 
lava  contains  little  steam,  or  is  in  such  a  condition  as  to 
allow  it  to  readily  escape,  as  it  does  from  the  lava  lake  of 
Kilauea,  it  cannot  gain  violence  enough  to  blow  the  liquid 
rock  into  the  form  of  volcanic  ash ;  but  if  for  any  reason 
its  escape  is  retarded,  it  gathers  force  until  escape  is  finally 
made  possible.^     Sometimes  a  crust  forms  over  the  crater, 

1  It  is  very  much  like  a  boiler  against  the  sides  of  which  steam  is  con- 
stantly pressing ;  but  the  boiler  is  strong  enough  to  stand  the  pressure  up 
to  a  certain  limit,  though  after  this  is  reached  an  explosion  takes  place. 
An  engineer  does  not  usually  allow  the  pressure  to  become  great  enough 
for  an  explosion,  but  every  now  and  then  permits  a  little  steam  to  escape, 
thus  relieving  the  pressure. 


VOLCANOES  857 

or  in  the  upper  part  of  the  neck,  and  this  may  grow  so 
thick  that  the  steam  cannot  blow  it  out,  and  then  the 
volcano  either  becomes  extinct  or  remains  dormant,  slowly 
gathering  force  enough  for  an  outbreak.  At  times  the 
plug  becomes  so  strong,  that  when  the  pressure  is  higli 
enough,  it  is  easier  to  break  a  new  opening  than  to  force 
the  plug  out,  as  it  is  easier  for  some  guns  to  explode  than 
to  drive  the  wadding  out.  Then  a  cone  may  be  built  at 
the  side  of,  or  near,  the  old  one.  Mt.  Shasta  is  a  double 
volcano  of  this  origin. 

EaKTH  QUAKES 

Before  and  during  a  volcanic  eruption,  the  movement  of 
the  steam  and  the  lava  sends  jars  through  the  rocks,  and 
the  earth  trembles,  and  sometimes  is  so  violently  shaken 
that  buildings  are  thrown  down.  There  may  be  hundreds 
of  such  earthquake  shocks  during  a  violent  outburst,  and 
the  destruction  of  life  is  very  great.  Any  jar  to  the  rocks 
will  produce  an  earthquake,  if  upon  the  land,  or  will  cause 
a  great  water  wave,  if  in  the  sea.^  Next  to  volcanic  causes, 
the  most  important  source  of  earthquake  is  the  breaking 
of  rocks.  As  they  move  along  the  fault  plane,  jars  pass 
out,  and  these  reaching  the  surface,  cause  earthquakes. 
In  many  cases,  as  in  Japan  in  1891,  the  breaking  of  the 
rocks  which  caused  the  shock  reached  to  the  surface,  and 
in  this  case  the  ground  was  cracked,  roads  rendered  im- 
passable, and  streams  interfered  with.  Since  both  volca- 
noes and  faults  are  associated  with  mountains,  earthquake 
shocks    are   frequent   only   in    those   places.     Hence   the 

1  A  miniature  shock  is  caused  when  gunpowder  is  exploded,  and  the  jars 
that  pass  over  a  frozen  pond  on  a  winter  night  are  similar  shocks. 


a58 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


greater  part  of  the  earth  is  free  from  them,  though  near 
Vesuvius,  on  the  western  side  of  South  America,  in  Japan, 
etc.,  earthquakes  are  exceedingly  frequent.     While  sHgJit 


Fig.  195. 

Effect  of  the  Japanese  earthquake  of  1891  upon  a  railway  bridge, 
was  shaken  from  beneath  the  track.    ' 


The  earth 


jars  are  not  uncommon  in  central  and  eastern  United 
States,  there  have  been  only  three  violent  earthquakes  in 
these  regions  since  they  were  settled;  one  during  the  last 
century,  near  Boston,  another  in  1812,  near  the  boundary 
of  Louisiana  and  Arkansas,  and  one  near  Charleston,  South 
Carolina,  in  1888.  During  the  same  period  there  have 
been  hundreds  in  Japan,  or  near  Vesuvius. 

The  earthquake,  being  a  jav  starting  from  some  centre,  extends 
outward  from  this  in  all  directions,  as  a  series  of  waves  of  rock  move- 


EARTHQUAKES 


359 


ment,  very  much  as  the  jar  caused  by  a  blow  upon  a  piece  of  iron 
passes  outward  to  both  ends  of  the  bar.  If  the  shock  started  from  a 
point,  the  waves  would  be  spherical,  and  if  the  rock  were  of  one  kind, 
they  would  advance  as  spheres  of  movement,  reaching  points  at  equal 
distances  from  the  place  of  origin,  or  focus,  at  exactly  the  same  time. 
In  reality  the  focus  is  not  a  point,  and  rocks  differ  in  character,  so 
that  the  waves  are  much  less  simple.  The  place  directly  above  the 
focus,  called  the  epicentrum,  is  tlie  place  where  the  shock  is  most  vio- 


FiG.  196. 

Diagram  to  illustrate  passage  of  earthquake  wave  from  a  point,  the  focus  (F) 
E,  epicentrum ;  I,  isoseismals. 


lent,  and  its  force  decreases  on  all  sides  from  this.  At  a  certain  dis- 
tance from  the  epicentrum,  in  all  directions,  the  shock  reaches  the 
surface  at  exactly  the  same  time.  If  the  waves  were  really  spherical, 
these  places  would  be  at  equal  distances  from  the  epicentrum,  but 
really  they  are  not.  The  somewhat  circular,  though  quite  irregular, 
lines  drawn  around  the  epicentrum,  and  passing  through  places  at  which 
a  shock  was  felt  with  the  same  intensity,  are  called  isoseismal  lines. 

To  understand  the  effects  of  the  earthquake  a  brief 
description  of  the  shock  which  devastated  Lisbon,  Portu- 
gal, in  1755,  may  be  introduced.  The  epicentrum  was  out 
to  sea,  not  far  from  Lisbon.  Without  other  warning  than 
a  sound  like  thunder  proceeding  from  the  ground,  there 
came,  almost  at  the  same  moment,  a  violent  shaking  of 
the  earth  which  destroyed  most  of  the  houses  in  the  city, 


360 


FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 


SO  that  in  six  minutes  60,000  people  were  killed;  and  fires, 
starting  immediately  after  this,  added  to  the  destruction. 
The  surface  of  the  ocean  fell,  then  rose,  and  a  wave 
rushed  upon  the  shore  to  the  height  of  50  feet  above  the 

ordinary  level.  Many 
people  who  had  es- 
caped, gathered  upon 
a  quay  or  stone  pier, 
which  extended  out 
into  the  water,  where 
they  would  be  free 
from  the  falling  build- 
ings ;  but  this  sank 
into  the  water,  carry- 
ing the  people  with 
it,  and  hence  Lisbon 
was  nearly  destroyed, 
while  a  large  part  of 
her  inhabitants  were 
exterminated.  This 
was  one  of  the  great- 
est of  earthquakes 
since  man  has  kept 
definite  records.  The 
shock  was  felt  in  the 
Alps  and  in  Germany, 
Sweden,  and  Great  Britain,  and  the  water  wave  reached 
across  the  Atlantic  as  far  as  the  West  Indies. 

When  occurring  in  the  open  country,  even  a  violent 
earthquake  is  not  destructive ;  but  if  the  epicentrum  is 
near  a  city,  the  falling  buildings,  and  the  fires  that  are 
caused,  add  greatly  to  the  destruction.     In  countries  like 


Fig.  197. 
Map  near  Charleston,  S.C,  showing  the  two 
epicentra  and  the  isoseisraal  lines  during 
the  shock  of  1888. 


EARTHQUAKES 


861 


Japan,  which  are  visited  by  frequent  shocks,  the  inhabit- 
ants build  low  houses,  so  constructed  that  they  are  not 
easily  thrown  down ;  yet  even  in  Japan  single  shocks  have 
destroyed  thousands  of  lives. 


Fig.  198. 
Giant  geyser  in  eruption,  Yellowstone  Park. 

Geysers.  —  Hot  springs  emit  hot  water,  which  flows  out  as  it  does 
from  any  spring,  with  the  exception  that  the  water  is  heated,  perhaps 
even  to  the  boiling  point.  In  some  cases  the  cause  for  this  heat  is 
no  doubt  intruded  lava  which  has  not  reached  the  surface ;  but  at 
other  times  its  cause  may  be  friction  along  the  sides  of  fault  planes, 
where  the  grinding  of  the  particles  produced  heat  for  the  same  reason 
that  a  knife  becomes  hot  when  held  against  a  dry  grindstone.  Per- 
haps the  water  sometimes  comes  from  such  a  great  depth  that  it  brings 
out  some  of  the  heat  that  exists  deep  in  the  crust. 

In  several  places  in  the  world,  but  especially  in  the  Yellowstone 
"Park,  Xow  Zealand,  and  Iceland,  the  hot  water  comes  forth  not  regu- 


362  FIRST  BOOK  OF  PHYSICAL   GEOGRAPHY 

larly,  but  intermittently,  forming  geysers.  There  are  periods  of  quiet, 
and  then,  perhaps  after  intervals  of  a  few  minutes,  hours,  days,  or 
even  months,  there  comes  an  eruption,  and  a  fountain  of  hot  water 
and  steam  rises  into  the  air  and  quickly  subsides.  Some  geysers  erupt 
at  regular  intervals,  but  others  irregularly.  The  cause  for  the  erup- 
tion is  the  presence  of  a  supply  of  heat,  which  warms  the  water  down 
in  the  tube  until  it  reaches  the  boiling  point,  expands  to  steam,  and 
expels  the  water  above  it  into  the  air.  Then  being  relieved,  it  re- 
mains quiet  until  the  heat  again  raises  the  temperature  of  the  water 
to  the  boiling  point,  when  another  eruption  occurs.  The  time  be- 
tween these  outbursts  will  therefore  depend  upon  the  length  of  time 
needed  to  heat  the  water  in  the  tube.^ 

1  A  geyser  may  be  artificially  made  by  warming  water  in  a  long,  nar- 
row glass  tube,  applying  heat  to  one  part  of  it.  If  the  teacher  wishes  a 
more  detailed  explanation  of  the  cause  of  geysers  he  will  find  it  either  in 
Le  Conte's  Elements  of  Geology,  or  in  my  Elementary  Geology,  p.  362. 


BOOKS   OF   REFERENCE 

Russell.  Lakes  of  North  America.  Ginn  &  Co.,  Boston,  Mass.,  1895. 
$1.65. 

Russell.  Glaciers  of  North  America.  Ginn  &  Co.,  Boston,  Mass,  1897. 
$1.65. 

National  Geographic  Monographs,  Vol.  I.  Articles  by  Powell, 
Russell,  Shaler,  Willis,  Gilbert,  Diller,  Davis,  and  Hayes.  American 
Book  Co.,  New  York,  1896.  $2.50.  Separate  papers  may  be  pur- 
chased singly. 

Merrill.  Rocks,  Rock- Weathering,  and  Soils.  The  Macmillan  Co., 
New  York,  1897.     $4.00. 

Geikie.  Ancient  Volcanoes  of  Great  Britain.  The  Macmillan  Co., 
New  York.    2  Vols.    1897. 

There  are  several  articles  of  general  interest  in  the  annual  reports  of 
the  United  States  Geological  Survey,  Washington,  D.C.  A  list  of  these 
can  be  obtained  from  the  Director. 


INDEX 


Absolute  humidity,  127. 

Absorption  of  heat,  57 ;  of  light,  47. 

Abyssal  fauna,  109. 

Accidents  in  river  valleys,  274. 

Adjustment  of  rivers,  281. 

Afterglow,  50. 

Age  of  earth,  236. 

Ages,  geological,  238. 

Air,  changes  in,  42;  composition  of, 
33;  effect  of  heat  on,  64;  height  of, 
40;  importance  of,  7,  32;  pressure 
of,  85;  vapor  in,  35. 

Alluvial  fan,  286. 

Altitude,  influence  of,  on  climate,  163; 
on  temperature  range,  77 ;  on  weath- 
ering, 252. 

Andromeda  nebula,  31. 

Anemometer,  101. 

Aneroid,  87. 

Animals,  distribution  of,  165;  of  land, 
178;  of  ocean,  167. 

Antarctic  circle,  20. 

Antarctic  zone,  67. 

Antecedent  rivers,  276. 

Anticline,  236. 

Anticyclonic  areas,  107. 

Anti-trades,  92. 

Arctic,  animals  of,  179;  climate  of, 
155. 

Arctic  circle,  19. 

Arctic  ice,  157,  190. 

Arctic  zone,  67. 

Argon,  33. 

Artesian  wells,  243. 

Ash,  volcanic,  352. 

Asteroids,  23. 

Atlantic,  currents  of,  216;  depth  of, 
198. 


Atmosphere,  32;  height  of,  40;  tem- 
perature of,  63. 
Aurora  Borealis,  54. 
Avalanches,  259,  295. 


Bad  Lands,  258. 

Barograph,  88. 

Barometer,  86. 

Barometric  gradient,  89. 

Barrier  reefs,  325. 

Barriers  to  spread  of  life,  185. 

Bars,  319. 

Base  level  of  erosion,  270. 

Beach,  230,  317. 

Beds,  233. 

Belt  of  calms,  91,  150. 

Bermuda,  animals  and  plants  of,  183. 

Blizzard,  120. 

Boulder  clay,  306. 

Butte,  253,  334. 


Calcite,  224. 

Carbonic  acid  gas,  33. 

Caverns,  246. 

Caves,  246. 

Centigrade  scale,  69. 

Charleston  earthquake,  358,  360. 

Chemically  deposited  rocks,  229. 

Chinook,  121. 

Circumpolar  whirl,  94. 

Cirro-cumulus  clouds,  139. 

Cirrus  clouds,  138. 

Cleavage  planes,  223. 

Climate,  149 ;  influence  on  weathering, 

252;  of  ocean,  62. 
Climatic  zones,  78, 149. 


363 


364 


INDEX 


Clouds,  135 ;  cause  of,  139 ;  forms  of, 
136 ;  materials  forming,  135. 

Coasts,  changes  in,  331;  form  of,  313; 
rising  of,  323 ;  sinking  of,  321 ;  wave- 
carved,  320. 

Cold  pole,  80. 

Cold  wave,  115,  160. 

Colors,  cause  of,  44,  47;  sunrise,  49; 
sunset,  49, 

Columns,  248. 

Combustion,  33. 

Conduction  of  heat,  59,  61,  65. 

Cone  deltas,  286. 

Cone,  volcanic,  345  ;  form  of,  352. 

Conglomerates,  230. 

Consequent  rivers,  278. 

Constructional  islands,  326. 

Continental  climate,  79. 

Continental  glacier,  308. 

Continental  shelf,  199. 

Continental  slope,  199. 

Continents,  9;  elevation  of,  12. 

Contraction  theory,  342. 

Convection  caused  by  heat,  59,  65. 

Coquina,  231. 

Coral  reefs,  168,  324. 

Cordilleras,  336. 

Coronas,  52. 

Crater,  345. 

Crevasses,  297. 

Crumpling  of  rocks,  233. 

Crust,  condition  of,  8,  220 ;  minerals  of, 
221;  movements  of,  234;  rocks  of, 
226. 

Crystalline  rocks,  221. 

Cumulus  clouds,  137. 

Currents  in  ocean,  215;  of  Atlantic, 
216. 

Cyclone,  tropical,  116. 

Cyclonic  areas,  107. 


Day,  cause  of,  13. 
Dead  seas,  174,  293. 
Deflection  of  currents,  67. 
Degrees,  4,  5. 
Deltas,  283. 
Denudation,  260. 


Depths  of  the  sea,  198. 

Deserts,  cause  of,  152. 

Dew,  130. 

Dew  point,  127. 

Diathermanous  bodies,  56. 

Dikes.  227,  233. 

Disintegration  of  rocks,  248. 

Dissected  rivers,  276. 

Distributaries,  284. 

Distribution   of  animals  and   plants, 

182;  of  man,  180. 
Divide,  263. 
Doldrums,  91. 
Dormant  volcanoes,  346. 
Drainage  area,  262. 
Dredge,  194. 
Drowned  rivers,  276. 
Dust  particles,  38;   effect  upon  heat 

rays,  66;  influence  on  color,  46. 

E 

Earth,  age  of,  236;  condition  of,  7; 
form  of,  3;  interior  of,  8;  move- 
ment of,  13 ;  origin  of,  27 ;  surface 
of,  9. 

Earthquakes,  357. 

Earth's  crust,  8,  9,  220. 

Electricity,  atmospheric,  52. 

Elements,  221. 

Epicentrum,  359. 

Equator,  6. 

Equatorial  drift,  216. 

Equatorial  zone,  67. 

Equinox,  16,  18. 

Erosion,  257 ;  by  rivers,  264. 

Ether,  22,  43. 

Evaporation,  35,  62,  126. 

Everglades,  332. 

Extinct  volcanoes,  346,  358. 


Fahrenheit  scale,  68. 
Faults,  235. 
Feldspar,  223. 
Floe  ice,  157,  190. 
Floodplains,  286. 
Focus  of  earthquake, 
Foehn,  121. 


INDEX 


365 


Fog,  133. 

Folding  of  rocks,  234. 
Fossils,  238. 
Fragmental  rocks,  229. 
Frigid  zones,  78;  climate  of,  155. 
Fringing  reefs,  325. 
Frost,  132;  action  of,  in  weathering, 
249. 

G 

Geological  ages,  238. 

Geysers,  242,  361. 

Glacial  deposits,  309. 

Glacial  lakes,  311. 

Glacial  period,  305. 

Glaciers,  294 ;  erosion  by,  257. 

Globigerina  ooze,  203. 

Gneiss,  232. 

Graham's  Island,  344. 

Granite,  227. 

Great  Basin,  340. 

Greenland,  climate  of,  159;  glaciers  of, 

300. 
Ground  moraine,  298. 
Gulf  Stream,  217. 


H 

Hail,  141. 

Halo,  51. 

Hawaiian  volcanoes,  349. 

Haze,  134. 

Heat,  absorption  of,  57;  conduction 
of,  59,  61,  65;  convection  caused  by, 
59,  65 ;  effect  of,  on  highlands,  62 ; 
effect  of,  on  land,  60,  64;  effect  of, 
on  ocean,  61;  latent,  62;  nature  of, 
55 ;  radiation  of,  57,  60 ;  reflection 
of,  55,61 ;  from  sun,  55 ;  of  evapora- 
tion, 62. 

Heat  equator,  80. 

Heat  lightning,  53. 

Hemispheres,  6,  10;  land  and  water, 

v., 

He.julaneum,  destruction  of,  346. 
Highlands,  temperature  of,  62,  79. 
High-pressure  areas,  106,  107. 
Horse  latitudes,  92. 
Hot  springs,  242,  245. 


Humidity,  37, 127. 
Hurricanes,  116. 
Hygrometer,  129. 


Icebergs,  304. 
Ice  fall,  297. 
Ice  of  Arctic,  157,  190. 
Igneous  rocks,  226. 
India,  climate  of,  154. 
Insular  climate,  78. 
Islands,  326 ;  oceanic,  13. 
Isobaric,  103. 
Isoseismal  lines,  359. 
Isothermal  chart,  80. 
Isothermal  lines,  79. 
Isotherms,  80. 


K 


Kaolin,  224. 
Keys,  314. 
Kilauea,  349. 
Krakatoa,  347. 


Lakes,  291 ;  formed  by  glaciers,  312. 

Lake  shores,  313. 

Land,  animals  of,  178;  effect  of,  on 
temperature,  72,  77 ;  erosion  of,  257 ; 
life  on,  174;  warming  of,  60. 

Land  breeze,  98. 

Land  hemisphere,  11. 

Landslides,  259. 

Latent  heat,  62. 

Lateral  moraines,  297. 

Latitude,  5,  6 ;  influence  upon  tem- 
perature range,  76. 

Lava,  351. 

Life,  barriers  to  spread  of,  185;  in 
fresh  water,  173;  on  the  land,  174; 
in  the  ocean,  165  ;  on  ocean  bottom, 
169,  191 ;  zones  of,  165. 

Light,  absorption  of,  47 ;  nature  of, 
43 ;  reflection  of,  44 ;  refraction  of, 
48;  selective  scattering  of,  48;  un- 
dulatory  theory  of,  43. 

Lightning,  52. 

Limestone  eaves,  24Q. 


366 


INDEX 


Lisbon  earthquake,  359. 
Littoral  faunas,  167. 
Longitude,  4,  5. 
Low  latitude,  0. 
Low-pressure  areas,  105,  107. 

M 

Magnetic  pole,  4,  53. 

Magnetism,  53. 

Man,  distribution  of,  180. 

Mangrove  swamps,  166,  324. 

Marshes,  323. 

Mature  valleys,  270. 

Mauna  Loa,  318,  349. 

Medial  moraines,  297. 

Mercurial  barometer,  87. 

Mercurial  thermometer,  68. 

Meridians,  4. 

Mesas,  334. 

Metamorphic  rocks,  231. 

Mid-Atlantic  ridge,  200. 

Migration  of  divides,  281. 

Mineral  springs,  244. 

Minerals  of  crust,  221. 

Mirage,  46. 

Mist,  135. 

Moisture  in  atmosphere,  126. 

Monocline,  236. 

Monsoons,  95. 

Monte  Somma,  346. 

Moon,  24. 

Moraines,  297. 

Mountain  breeze,  98. 

Mountain  system,  336. 

Mountain  valleys,  340. 

Mountains,  cause  for  coolness  on,  63; 
cause  of,  342;  climate  of,  79;  de- 
struction of,  340;  development  of, 
337;  nature  of,  335. 

Mountainous  irregularities,  12. 

Mud  tlow,  351. 


N 

Natural  bridge,  247. 
Natural  levee,  287. 
NebulfE,  31. 
Nebular  Hypothesis,  27. 


Ne've,  295. 

Niagara,  289, 290. 

Night,  cause  of,  13. 

Nimbus  clouds,  137. 

Nitrogen,  33. 

Northeast  storms,  113. 

Norther,  120. 

Northern  hemisphere,  6,  10. 

Northern  lights,  54. 

North  Poiar  zone,  67. 

North  Pole,  4. 

North  Temperate  zone,  67. 

Nunataks,  301. 


Oblate  spheroid,  6. 

Ocean,  animals  in,  167;  area  of,  187; 
currents  of,  215;  depth  of,  12,  198; 
importance  of,  7,  187;  intiuence  of, 
on  climate,  163;  intiuence  of,  upon 
temperature  range,  72,  77 ;  life  in, 
165;  movements  of,  205;  salt  of, 
188 ;  temperature  of,  189 ;  warming 
of,  61. 

Ocean  basins,  9. 

Ocean  bottom,  animals  of,  169,  191; 
materials  of,  203;  temperature  of, 
195 ;  topography  of,  200. 

Oceanic  climate,  78. 

Oceanic  islands,  13. 

Old  river  valleys,  273. 

Opaque  bodies,  47. 

Organic  rocks,  229. 

Ox-bow  cut-offs,  287. 

Oxidation,  33. 

Oxygen,  33. 


Peaks,  336. 
Pelagic  faunas,  171. 
Periodical  winds,  95. 
Plains,  332;  treeless,  335. 
Planetary  winds,  90. 
Planets,  23. 

Plants,  distribution    of,  165;    of  the 
land,  174 ;  aid  of,  in  weathering,  250. 
Plateaus,  334. 
Pocket  beaches,  318. 


INDEX 


367 


Poles,  4. 

Pompeii,  destruction  of,  346. 

Pot  holes,  266. 

Prairies,  335. 

Pressure  of  air,  85 ;  change  in,  88. 

Prevailing  westerlies,  93. 

Profile  of  equilibrium,  271. 

Promontories,  330. 

Psychrometer,  129. 

Pumice,  352. 


Quartz,  222. 


Q 


R 


Radiant  energy,  44,  55. 

Radiation  of  heat,  57,  60 

Rain,  140;  in  cyclones,  113;  distribu- 
tion of,  145 :  erosion  by,  258 ;  meas- 
urement of,  144;  nature  of,  144. 

Rainbow,  51. 

Rainfall  charts,  147. 

Rain  gauge,  144. 

Ranges,  336. 

Red  clay,  204. 

Reefs,  324. 

Reflection  of  light,  44 ;  of  heat,  55,  61. 

Refraction  of  light,  48. 

Rejuvenated  rivers,  274. 

Relative  humidity,  127. 

Residual  soil,  257. 

Revived  rivers,  274. 

Revolution,  15 ;  effect  of,  on  sun's  heat, 
67. 

Ridges,  336. 

Right-hand  deflection,  67. 

Rivers,  course  of,  278;  erosion  by,  259, 
263. 

River  system,  262. 

River  valleys,  261;  accidents  to,  274; 
effect  of  glaciers  upon,  311 ;  history 
of,  208. 

River  work,  264. 

Rocks,  of  the  crust,  226 ;  disintegra- 
tion of,  248 ;  igneous,  226,  351 ;  met- 
amorphic,  231 ;  position  of,  232;  sedi- 
mentary, 228;  volcanic,  227. 

Rotation,  13;  effect  of,  on  sun's  heat, 
66. 


S 

Sahara,  temperature  of,  73. 

Salt  lakes,  174,  293. 

Salt  marshes,  166,  323. 

Salt  of  ocean,  188. 

Satellites,  24. 

Saturation  of  air,  36,  126. 

Scattering  of  light  rays,  48. 

Sea  breeze,  97. 

Sea  cliffs,  314. 

Sea  ice,  157,  190. 

Sea  level,  8. 

Sea  shores,  313. 

Seasonal  temperature  change,  74. 

Seasons,  cause  of,  17,  21 ;  effect  of,  on 

daily  temperature  change,  71. 
Sedimentary  rocks,  228. 
Selective  scattering,  48. 
Serpula  atolls,  327. 
Shores,  313. 
Sirocco,  120. 
Snow,  141. 

Snowfall,  distribution  of,  148. 
Snow  field,  294. 
Snow  line,  294. 
Soil,  formation  of,  256. 
Solar  system,  22  ;  symmetry  of,  27. 
Sounding  machine,  193. 
South  Pole,  4. 
South  Polar  zone,  67. 
South  temperate  zone,  67. 
Southern  hemisphere,  6,  10. 
Space,  22. 

Springs,  242;  mineral-bearing,  244. 
Stalactites,  248. 
Stalagmites,  248. 
Stars,  26. 

Steam  in  volcano,  351. 
Stellar  system,  26. 
Storm  winds,  119. 
Storms,  102. 
Strata,  229,  234. 
Stratification,  233. 
Stratified  rocks,  234. 
Strato-cumulus  clouds,  138. 
Stratus  clouds,  136. 
Submerged  rivers,  276. 
Sun,  22 ;  heat  from,  55 ;  position  of,  15 
Sunlight,  measurement  of,  52. 


368 


INDEX 


Sunrise  colors,  49. 
Sunset  colors,  49. 
Sunshine  recorder,  52. 
Superimposed  rivers,  280. 
Swamps,  292. 
Syncline,  230. 
System  of  mountains,  336. 


Talus,  255. 

Temperate  zones,  78 ;  climate  of,  160. 

Temperature  in  anticyclones,  115;   in 

cyclones,  114 ;  daily  change  of,  70 ; 

of  earth's  interior,  8;    of    earth's 

surface,  70;   of  ocean,  189,  215;   of 

ocean  bottom,  195 ;  seasonal  range 

of,  74 ;  extremes,  84. 
Terminal    moraines,  298;    of  glacial 

period,  308. 
Thermograph,  69. 
Thermometers,  68 ;  dry  and  wet  bulb, 

129. 
Thunder,  53 ;  storms,  121. 
Tides,  210;  cause  of,  211;  effects  of, 

213;   nature  of,  210. 
Till,  306. 

Timber  line,  176,  341. 
Time,  reckoning  of,  15. 
Time  scale,  238. 

Topography  of  ocean  bottom,  200. 
Tornadoes,  124. 
Torrid  zone,  67. 
Trade-wind  belt,  152. 
Trade  winds,  91. 
Treeless  plains,  335. 
Tropical  cyclone,  116;  zone,  67,  78; 

climate  of,  150. 
Tropics,  19. 
Typhoons,  116. 


U 

Underground  water,  242,  259. 

Undertow,  209. 

Undulatory  theory  of  light,  43. 


United  States,  climate  of,  161. 
Universe,  22,  25. 


Valley  breeze,  98 ;  glaciers,  294. 

Valleys  in  mountains,  340. 

Vapor,  35,  126;  effect  upon  heat 
waves,  64. 

Vernal  equinox,  18. 

Vesuvius,  345. 

Volcanic  ash,  352. 

Volcanic  necks,  354. 

Volcanic  rocks,  227. 

Volcano,  section  of,  227,  233. 

Volcanoes,  birth  of,  344 ;  cause  of, 
356;  differences  in,  356;  distribu- 
tion of,  355;  of  Hawaii,  349;  mate- 
rials erupted  from,  351 ;  on  moon,  25. 

W 

Water  in  earth,  240 ;  effect  of,  on  tem- 
perature, 72,  77 ;  in  volcano,  351 ; 
importance  of,  in  weathering,  248. 

Waterfalls,  288. 

Water  hemisphere,  11. 

Water  parting,  263. 

Water  vapor,  35. 

Wave-carved  shores,  320. 

Waves,  205. 

Weather  changes,  102 ;  map,  102. 

Weathering,  248 ;  effects  of,  254. 

Wind  vane,  101 ;  waves,  205. 

Winds,  85;  in  anticyclones,  112,  119; 
in  cyclones,  112,  119;  erosion  by, 
258;  periodic,  95;  planetary,  90; 
storm,  99,  112,  119;  velocity  of,  99. 


Year,  16. 

Young  valleys,  269. 


Zones,  climatic,  67,  78. 


Elementary  Physical  Geography^ 


BY 


RALPH   STOCKMAN   TARR,   B.S.,    F.G.S.A., 

Professor    of  Dynamic   Geology    and    Physical    Geography    at    Cornell   University  i 
Author  of  ^' Economic   Geology  of  the  United  States,"  etc. 

Fifth  Edition,  Revised.     lamo.    Cloth.    $1.40  net. 


"  There  is  an  advanced  and  modernized  phase  of  physical  geography,  how- 
ever, which  the  majority  of  the  committee  prefer  to  designate  physiography, 
not  because  the  name  is  important,  but  because  it  emphasizes  a  special  and 
important  phase  of  the  subject  and  of  its  treatment.  The  scientific  investi- 
gations of  the  last  decade  have  made  very  important  additions  to  the  physio- 
graphic knowledge  and  methods  of  study.  These  are  indeed  so  radical  as 
to  be  properly  regarded,  perhaps,  as  revolutionary." 

"The  majority  of  the  Conference  wish  to  impress  upon  the  attention  of  the 
teachers  the  fact  that  there  has  been  developed  within  the  past  decade  a  new 
and  most  important  phase  of  the  subject,  and  to  urge  that  they  hasten  to 
acquaint  themselves  with  it  and  bring  it  into  the  work  of  the  school-room 
and  of  the  field."  —  Report  of  Geography  Conference  to  the  Committee  of  Ten. 


The  phenomenal  rapidity  with  which  Tarr's  Elementary  Physical  Geography 
has  been  introduced  into  the  best  high  schools  of  this  country  is  a  fact 
familiar  to  the  school  public.  The  reason  should,  by  this  time,  be  equally 
familiar  —  the  existence  of  a  field  of  school  work  in  which,  until  the  appearance 
of  Tarr's  book,  there  was  not  a  single  adequate  or  modern  American  text- 
book. That  such  a  field  did  exist,  is  simply  shown  by  the  paragraphs  reprinted 
above.  The  adoption  of  the  book  in  such  important  high  schools  as  those  of 
Chicago,  and  the  expressions  of  approval  from  representative  New  England 
schools,  will  indicate  how  well  the  field  has  been  covered. 

Tarr's  High  School  Geology,  uniform  with  Elementary  Physical  Geo- 
graphy, has  attained  wide  use  since  its  publication  in  February. 


THE    MACMILLAN    COMPANY. 

NEW  YORK.  CHICAGO.  SAN    FRANCISCO. 


Elementary   Physical   Geography, 

By  PROF.   RALPH   S.    TARR. 


From  those  who  use  it  in  New  England  Schools. 

Dr.  F.  E.  Spaulding,  Siipt.  of  Schools,  Ware,  Mass.  Tarr's  Physical 
Geography  has  been  in  use  in  the  Ware  High  School  since  September  last. 
We  regard  it  as  incomparably  superior  to  any  other  book  on  the  subject. 
Previous  to  its  publication,  this  most  important  and  interesting  department 
of  science  was  seriously  handicapped  by  the  lack  of  a  text  suitable  for  use  in 
secondary  schools.  Now  no  other  subject  taught  in  a  high  school  can  boast 
of  a  more  adequate  text  than  Tarr's  Physical  Geography. 

C.  A.  Byram,  Principal,  High  School,  Pittsfield,  Mass.  We  have  used 
Tarr's  Physical  Geography  now  for  several  months,  and  like  it  very  much.  It 
is  both  simple  and  scientific,  while  the  make-up  of  the  book  is  most  pleasing. 

Miss  H.  A.  Luddington,  State  Normal  School,  Fitchburg,  Mass.  I  am 
very  glad  to  express  my  great  appreciation  of  the  value  of  Tarr's  Physical 
Geography.  I  rely  upon  it  for  clear  statement  and  full  illustration  of  all 
important  topics  in  Physical  Geography.  As  an  aid  to  field-work  in  Physiog- 
raphy, I  know  of  no  book  so  helpful. 

Miss  Maud  L.  Williams,  High  School,  Northafnpton,  Mass.  I  have  never 
been  so  highly  satisfied  with  any  text-book  as  I  am  with  Tarr's  Physical 
Geography. 

Alfred  0.  Tower,  Principal,  Lawrence  Academy,  Groton,  Mass.  I  used 
Tarr's  Physical  Geography  with  my  class  last  term,  and  consider  it  by  far  the 
best  work  published  on  this  subject  for  High  School  or  Academy  use. 

J.  C.  Simpson,  Superintendent  of  Schools,  Portsmouth,  N.  H.  For  the 
past  year  we  have  been  using  Tarr's  Physical  Geography  in  our  high  school, 
and  as  a  text-book  basis  for  an  advanced  study  of  geography  by  a  class  of 
teachers  from  our  grammar  grades.  In  both  uses  the  book  has  given  the 
highest  degree  of  satisfaction.  It  seems  to  me  to  touch  the  ideal  of  modern 
geographic  instruction  more  nearly  than  any  other  book  published. 

Miss  Atta  L.  Nutter,  Miss  Wheeler's  School,  Providence,  R.  I.  Tarr's 
Physical  Geography  we  are  more  and  more  pleased  with  as  the  work  pro- 
gresses. 

G.  W.  Flint,  Principal,  High  School,  Collinsville,  Ct.  Tarr's  Physical 
Geography  has  proved  highly  satisfactory  in  the  class  room  to  both  pupils  and 
teacher.  The  subject  matter  of  the  work  and  the  arrangement  and  treatment 
make  it  the  best  text-book  on  Physical  Geography  that  I  have  yet  seen. 

F.  A.  Verplanck,  Superintendent  of  Schools,  So.  Manchester,  Ct.  We  use 
Tarr's  Physical  Geography  with  a  class  of  Grammar  School  pupils  and  find 
it  very  satisfactory.     I  believe  it  is  the  best  book  on  the  market  to-day. 


Elementary  Physical   Geography. 


BY 

RALPH   STOCKMAN   TARR,   B.S.,  F.G.S.A., 

Professor  of  Dynamic  Geology  and  Physical  Geography  at  Cornell  University. 
Fifth  Edition,  Revised.     i2mo.     Half  Leather.    $1.40. 


A  PARTIAL  LIST  OF  SCHOOLS  USING  THIS  WORK. 

Normal  School,  Fitchburg  . 

Mass. 

High  School,  Attica     .        .        .        . 

Ind 

"             "        Framingham      . 

" 

South  Bend   . 

" 

"             "         Salem 

*' 

"          «*         Hanover 

'« 

"             "        North  Adams     . 

" 

"          "         Connersville  . 

'« 

High  School,  Amherst 

'« 

"          "        Jackson 

Mich. 

"        "        Bridgewater . 

«* 

State  Normal  School,  Ypsilanti  . 

" 

"        Natick  .... 

" 

The  Fourteen  High  Schools,  Chicago 

III 

"        "        North  Attleboro    . 

'* 

including  those  at 

"         "        Northampton 

" 

Hyde  Park     . 

«« 

Pitt>,field 

" 

So.  Chicago   . 

«' 

"         "        Springfield     . 

" 

Englewood     . 

" 

Ware      .... 

" 

Morgan  Park 

" 

"         "        Weymouth    . 

{< 

Oak  Park        • 

" 

Howe  School,  Billerica 

" 

Aurora  . 

" 

Lawrence  Academy,  Groton 
Training  School,  Holyoke  . 

" 

Lewis  Institute,  Chicago      . 

'« 

«' 

Armour  Institute,  Chicago  . 

" 

Tabor  Academy,  Marion     . 

" 

Acad,  of  N.  W.  Univ.,  Evanston 

«' 

Worcester  Academy,  Worcester 

" 

High  School,  Carthage 

" 

Morgan  School,  Clinton 

Conn 

"          "         Princeton 

'« 

High  School,  Collinsville     . 

" 

Pittsfield 

'« 

Wesleyan  Univ.,  Middletown     . 

" 

"          "         Waukegan      . 

" 

Normal  School,  New  Britain 

«' 

Lake  Forest  Academy 

" 

Hillhouse  High  School,  New  Haven 

•     " 

Morgan  Park  Academy,  Morgan  Par 

<       " 

Williams  Mem.  Inst.,  New  London 

" 

Monmouth  College,  Monmouth  . 

" 

High  School,  South  IVIanchester 

" 

High  School,  Kansas  City  .         . 

Mo. 

East  Greenwich  Academy  . 

R.I. 

Hannibal 

" 

Mr.  Diman's  School,  Newport    . 

" 

"          "         Burlington      . 

la. 

Miss  Wheeler's  School,  Providence 

" 

Cedar  Falls    . 

Normal  School,  Johnson      . 

Vt. 

"          "         Davenport      . 

*• 

High  School,  Barre     . 

" 

"          "         Marshalltown 

" 

"        "        Brandon 

" 

"          "         Iowa  City 

" 

«'         "        Portsmouth    . 

•   ^-.P- 

"          "         Grand  Rapids 

^vis. 

"         "        Wolfboro        .         . 

"          "         Randolph 

Seminary,  Kent's  Hill 

Maine 

Marshall 

Minn 

Teachers'  College         .         .         .     N 

ew  York 

Shattuck  School,  Faribault  . 

" 

ColumlJia  Grammar  School 

'< 

State  Normal  School,  Winona     . 

«« 

Quincy  School,  Poughkeepsie     . 

" 

High  School,  Lincoln 

.     Neb. 

Colgate  Academy,  Hamilton 

" 

Agricultural  College    . 

N.  D. 

Manual  Training  School, 

Normal  School,  Los  Angeles 

.       Cal. 

Brooklyn    . 

" 

"          "         San  Jose 

" 

Union  School,  Warsaw 

" 

"         Chico      . 

« 

Academy,  Middletown 

" 

High  School,  Riverside 

'* 

Stevens  School,  Hoboken    . 

.     N.J. 

"          "         Sacramento    . 

« 

Collegiate  Institute,  York   . 

.        Pa. 

"          •«         Stockton 

" 

Columbia  Academy,  Washington 

.     D.  C. 

"         "        St.  Helena     . 

" 

Public  Schools,  Xenia 

.      Ohio 

"          "         Redlands 

" 

"           "         Akron 

" 

"          "         Petaluma 

" 

High  School,  Newark 

" 

Visalia    . 

(( 

Prep.  Dept.  Univ.,  Wooster 
Webb  School,  Bell  Buckle  . 

*' 

••         "        Selma     . 

" 

.    Tenn. 

"          "         Tulare    . 

'« 

Citronelle  College 

.       Ala. 

Univ.  of  Pacific,  College  Park    . 

" 

State  Normal  School  . 

.       Ind. 

Gartner  Seminary,  Irvington 

«« 

High  School,  Westfield       . 

" 

Los  Angeles  Academy 

" 

••        "       Marion.        .        . 

" 

Throop  Polytechnic  Inst.,  Pasadena 

«< 

"        "       Peru      . 

" 

Trinity  School,  San  Francisco    . 

<i 

'•        "       Pendleton      . 

*        " 

ELEMENTARY  GEOLOGY. 


BY 


RALPH   STOCKMAN   TARR,   B.S.,    F.G.S.A., 

Professor  of  Dynamic    Geology  and    Physical    Geography  at   Cornell   University; 
Author  of  ^^ Economic   Geology  of  the  United  States"  etc. 


i2mo.    Cloth.    486  pp.    Price  $1.40  net. 


COMMENTS  OF  THE  PRESS. 

*•'•  We  do  not  remember  to  have  noted  a  text-book  of  geology  which 
seems  to  so  go  to  the  heart  of  the  matter."  —  Phila.  Evening  Bulletin. 

"The  author's  style  is  clear,  direct,  and  attractive.  In  short,  he  has 
done  his  work  so  well  that  we  do  not  see  how  it  could  have  been  done 
better."  —  Jonrnal  of  Pedagogy. 

"  It  is  far  in  advance  of  all  geological  text-books,  whether  American 
or  European,  and  it  marks  an  epoch  in  scientific  instruction." 

—  The  AiJierican  Geologist. 

"  The  student  is  to  be  envied  who  can  begin  the  study  of  this  deeply 
interesting,  fascinating  subject  with  such  an  attractive  help  as  this 
text-book."  —  Wooster  Post-Graduate. 

"The  Geology  is  admirably  adapted  for  its  purpose  —  that  of  a  text- 
book." —  Brooklyn  Standard  Union. 

"  So  admirable  an  exposition  of  the  science  as  is  found  in  this  book 
must  be  welcomed  both  by  instructors  and  students.  The  arrange- 
ment of  facts  is  excellent,  the  presentation  of  theory  intelligent  and 
progressive,  and  the  style  exceedingly  attractive."  —  N.  Y.  Tribune. 


THE   MACMILLAN    COMPANY, 

66  FIFTH    AVENUE,  NEW   YORK. 


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